Okie doke. It appears I'm stuck with writing this one up. I wrote a lot of comments because the remedies associated with your problem typically lack the certainty I associate with an "answer". That said, you have a mystery pipe. Multiple mystery pipes in fact. You haven't mentioned what kind of conduit it is, but it matters whether the pipe is plastic or metal, embedded in a slab, and even what type of pipe if plastic.
First thing to get out of the way is safety. Blind fishing is a 2 person job, partly because you need someone to look and listen, and partly because you may need someone to call EMS. The tool typically used to fish pipes is a "fish tape" and you can get versions made of metal and fiberglass. If you're pushing a fish tape to a mystery location, there is a chance you'll end up somewhere live. A fibreglass fish tape poses little risk in this situation, whereas a metal one in a metal pipe will cause a flash when it shorts something to ground, and if it touches something with a high current limit, the results can be spectacular and in a very bad way. A box or panel or parts of them can literally be blown right off the wall. A metal fish tape in a plastic conduit will instead be energized and if you're not lucky enough it is touching ground somewhere, you become the path to ground and because of the way a fish tape must be gripped to feed it, there is a high risk of "lockup" where a voltage is sufficient to keep your muscles flexed and prevent you from letting go. As far as I've seen, lockup on 240V and lower is rare, but you could still receive a bad shock, or if you're not lucky, be electrocuted. If you're in a large building, a pipe could have been abandoned because it was misconnected to a different system which may have a higher voltage.
So you need a fibreglass fish tape, right? Weeeeeeeellll it's not that simple. Fibre tapes are less strong, durable and rigid, and they dampen the noises they make in some ways, so if you have a metal pipe and you run a fish tape all the way into it and then ram the fish tape so it ripples and hits the side of the pipe, with a metal fish tape you can often sense some vibration on the whole length of the pipe, with a fibre tape, much less so. If the pipe is plastic it'll be harder to hear either way, and if it's embedded in concrete or insulation you won't hear it this way anyway. The other way the tape can make noise is by the ramming end, and again, metal boxes and pipe tend to resonate better. One other thing to note, locating pipes falls into hard use for fish tapes, and the name brand tapes like GREENLEE in particular vastly outperform cheaper generic fish tapes for hard use in my experience. My comments about fish tape durability are about GREENLEE tapes, and I have absolutely had to buy the odd cheapo tape and wreck it doing something mickey mouse easy for a GREENLEE, and have also bought a cheapo tape when I was absolutely sure whatever tape I used would be wrecked. You'll have to make your best guess what's right for you, but that's the background.
Sometimes a "just in case" pipe that is not later used is simply taped off in duct tape or not even capped. In this case the fish tape can be pushed through the tape and typically it's possible to make a fair bit of noise by rattling it if it happens to land in a void or run into drywall or steel stud. A fibreglass fish tape can be wrecked trying to push through 3 layers of decent duct tape, 2 layers if the tape was put on well. A steel fish tape can take probably 10 or 20 times the force and can usually be straightened if it kinks, especially if you were using good technique and the kink is limited. If you push a fishtape into a mystery void, there is a chance it will bind or jam or hook on something and you will be unable to withdraw it unless you find the other end of the pipe, potentially costing you valuable fish tape length and occassionally it can't be removed from the other end, sacrificing the pipe. I could write a few pages on how to extract a stuck fish tape, but this will be bulky enough anyway so we'll cross that bridge if we come to it.
Soooo, strongly preferring a metal fish tape I would approach your problem. Is it possible to eliminate the chance of fishing into something live? If it's an apartment building or has adjoining units, there could be multiple power systems, but if the buiding is stand alone, I can check for any additional pipes coming into the service side of the main panel (there should be none) and look at the load side of the panel. If I turn off the main disconnect, is there any hole big enough for a fish tape that leads to the service side? Better securely block that. Once the main breaker is off, I can use a metal tape without fear with the caveat that if my fish tape gets jammed in something and I can't extract it, I might have to check every live box in the house just to be sure before re-energising. On the other hand, if my fish tape ended up in an existing box, I'd want to know about it, so guess what? I'll strongly consider opening every reasonably accessible box in the area (or because you have multiple pipes to check, possibly the whole house. I'd also check above the suspended ceilings for any end points to watch for. Conveniently for an electrician, mystery pipe recovery is typically done during initial construction, before energisation, and typically before an area is closed up.
Hmmm so how do I feed the fish tape? Well to start with, your fish tape either does or doesn't have a leader on it. If it doesn't, you want a good head to fish with, so look up how to put a head on a fish tape and preferably use a torch to make the tight bends. You want a relatively low chance of hooking on an edge if you come out of pipe and have to extract your fish tape, so you can tape the back of the loop into a nice cone shape with tightly wound tape. If you're using a leader, the square back edge can catch too, especially if something presses it to the side, so it's wise to tape a cone onto that.
Ok so now I can put the end of the fish tape into the pipe and start feeding it in, noting any bumps I have to jiggle to get past or things that feel like constrictions or bends I'm pushing through. I'm thinking about how I'm applying force to the tape so the force translates well to feed it down the pipe rather than bending it or kinking it. I usually have one hand about a foot out from the pipe to produce the bulk of the pushing force and the other hand with my fingers a few inches out to support the curve from my pushing angle into the pipe. This also gives you additional sensitivity to how much you're "helping" when it arrives at a new curve or restriction.
Ok so I've fed the fish tape in and what I'm pushing against feels like pushing on a brick wall. This may mean I've hit an end of the pipe that is taped off in concrete, I've reached a box and I'm wedged in a corner, I've hit the end of a pipe that has been capped off, like with a PVC cap that may or may not be glued, or the bends in the pipe are pushing the tip of the fish tape sideways and it is caught on some slight ridge like a coupling or connector not done up all the way or a ridge in core line (smurf tube in countries where it's blue). Another worst case this could indicate is that some fastener has been driven through the pipe.
At this point, I'll strongly consider adding a butterfly to my fish tape and fishing the pipe several times with lube. Especially if being jammed on a coupling or just friction could be the issue. The butterfly helps the tip of the fish tape bounce past obstructions at the same time as spreading lube on the inside of the pipe. To make a butterfly, take pulling twine or baling twine and run loops through the loop of the fish tape so you have one end of the twine and two 10cm loops sticking out on each side (one 10cm loop each side for 1/2" pipe). carefully and tightly tie the ends to each other with a square knot, preferably sitting inside the loop of the fish tape, and then tie the opposite loops tightly to eachother with a square knot, one on each side of the tape. Then pack a bunch of lube into the pipe, lube up the butterfly and try fishing the pipe again. The tip of the fish tape won't be able to rattle, but if you can get to the destination, it can still tap.
I get it as far as I can and arrange the part around me so I can pull it back 8 inches and repeatedly ram it against the end of the pipe. I note if it feels soft or hard, and anything else I can notice in the ten or twenty minutes I do the tap tap taparoo while my apprentice looks and listens. Then we switch jobs and try again. While you're tapping, you may as well be trying to feed it farther, so you can try tapping and bouncing with timing to make the end jiggle in case it has just caught a ridge on the inside of the pipe or something. Sometimes in your efforts to make noise you simply get the fish tape in another few metres, which might be the information needed to look in the right location.
If what I'm pushing into seems soft like duct tape, I may try twisting the fish tape. As long as it hasn't gone through too many bends, with a metal fish tape, you can hold an S shape in the part sticking out and turn it like an augur. If it turns easily it's a straight pipe, so you have some information. If the pipe has bends, the fish tape will act like a spring, storing force as you try to twist it until it flips over with a loud snap. You can wreck a fish tape this way, but if you're good you can also do this many many times before wearing a tape out. What you're trying to take advantage of is that up to a threshold bending a metal it acts like a spring before it reaches the greater amount of force that will permanently deform it.
If you have good 9 inch linesman pliers, you can also attempt to make loud noises or bounce past obstructions by rippling the fish tape. Feed the fish tape in as far as you can, and keep feeding it until you can't push it any harder by hand. Without releasing the pressure, grab the fish tape an inch or two out of the mouth of the pipe with your pliers, gripping the pliers as far from the jaw as you can for maximum grip strength and keep pushing. Either the fish tape will smash through, or it will have to absorb the extra length by rippling back and forth inside the pipe. By either applying more force or angling the pliers back and forth, you can often get the fish tape to snap back and forth, which can cause the tip to bounce off the side in addition to making noise. If you make a mistake angling the pliers you will kink the tape. For upward or sideways facing pipes, you can put your elbow on your hip and push with your legs, using your arms only for control, positioning and grip strength. A 150lb person can typically push a metal tape through 4 or 5 layers of duct tape or through a forgotten plastic knockout with this method, although it can be exhausting. A fibreglass tape will almost certainly be ruined. You may kink a metal tape this way, especially if you slip or grip it too far out of the pipe with your pliers, but it is typically recoverable.
Some box types meant for plastic pipe in concrete can have a pipe installed without the knockout being removed. If I think this is a possibility, I'll be sure to listen for tapping at those boxes, or even simply smash the knockouts with a flat blade as there is no risk to losing the knockouts once concrete is poured. If a plastic box of this type is not poured into concrete, you cannot lose the knockouts, so you can't preemptively do this. If you however believe you have this problem with a screw mounted plastic box, you risk unmounting the box smashing it out this way, although I've had success using a 90 degree and flexible cord attachment for a Dremel to drill a hole in a knockout and either rout it out with a side cutter like a drywall bit or put a screw into the hole and use the screw to pry the knockout out. Some boxes are too thick to pry out and you're back to routing it out.
Ok so what if your fish tape is going a suspiciously long way or feels too unobstructed? First, stop. You may have entered a void space and the fish tape may be bouncing and coiling into a knot that you won't be able to pull out. Mark the fish tape with tape so you know how far you got and pull it out. Measure if you think the information will help. Feed it in again, this time being conscious and ready to sense when it exits the pipe. You can shake it vigorously by feeding it back and forth 60cm as fast as you can and carefully continue to feed it a few feet at a time while shaking vigorously. This has a really good chance of producing an audible noise, just don't go too far and make a knot.
Other than making sound directly with the fish tape there are a few options. You can project sound into the pipe, and it carries fairly well, but you need to isolate the person listening for the other end as much as possible from hearing the source of the sound directly. If you're a few floors apart you can yell or whistle in to the pipe, or use an actual whistle as Ed Beal suggests or a device like an electronic rape whistle to produce a noise at the other end. If you have some core line or EMT available you can use it as a trumpet to blow into the pipe as suggested by NoSparksPlease. These methods do not work very well with a capped pipe, so the airflow test with air bottle or vacuum before this point can be valuable, or the puncturing of duct tape if applicable. You can potentially also create sound from the tip of the fish tape by building a piezo buzzer onto the head of your fish tape, as they are the loudest thing I'm aware of that will fit in a pipe. You could also take apart a skinny electronic rape whistle for parts and likely build something that will fit in a pipe.
Alright, camera time! Or more accurately, camera time may come before extreme fish tape methods depending on how far into the pipe I am, what size the pipe is and how far away my cameras are. I have one camera that is best of the the cheap versions of a pipe fishing camera. It only cost me $40 but I would have to wait up to a year for another similar sale, so I still treat cost of replacement as $100. It'll fish 1/2" pipe but it's hard on it and even 3/4" pipe inevitably wears it out. I estimate cost of one inspection in a 1/2" pipe to be $2. On the other hand, if the problem is within 15m range, you'll know exactly what is wrong. Screw through pipe? Screw dropped in pipe? Silt or sawdust or concrete? A kink in a bend or a gap from a coupling or a ridge in a transition coupling are all plain as day. You may be able to tell that you've entered a box or a void, although figuring out which way is up is hard. If you know which way was up when you inserted the camera though, you can say "Ok it turns right so it's going east then it turns up so it's going south and then it turns left and ends so it's probably stubbing up into the floor above right about... here! Oh, I see it got blocked by a pre cast wall so they just ran a new pipe.". Another option is to get cheap USB inspection camera from AliExpress, and I've had one work with my phone with its own 10 foot cable plus a 10 foot extension cable. It's not rigid enough to fish with alone, but it's skinnier than a rigid inspection camera, so I use it for looking in tiny holes or sacrificial uses. Because it twists with the fish tape you feed it with, guessing which way a pipe turns is much more difficult. Both cameras are branded as waterproof and I have wrecked both types by fishing a wet pipe after it had been worn out enough. Even once a fish tape or string has been gotten through a pipe if the quality of pipe is in question (you should be able to pull the correct size "mouse" or "clean out" through the pipe in both directions, which will also clean out loose solids like bits of concrete and screws.), you may still need it to be camera checked to be sure if you pull something through by force you aren't pulling through an unacceptable pinch or into a pipe with a screw through it. If the cameras you can afford can't cut it, you may have to rent a camera or hire someone who has a high-end one.
While we're on the topic of things you may have to rent or hire, depending on the type of conduit and construction, a toner may be of use. The type you use for this has a transmitter that energizes something conductive with a signal and a receiver that looks like a metal detector, sometimes with a horizontal bar instead of a round head. If it is a metal conduit, you can't sense a wire through it, but you can energize the conduit itself, which will light up everything grounded. If you have mostly wood construction, this can be a good way to trace a metal pipe. If you have steel stud or too much rebar too close it will find the signal everywhere. If the pipe is plastic, you can energize the fish tape and easily sense it without much interference. If sections of the pipe are plastic, you can attach an insulated wire and feed it in with your fish tape and energize that. You won't be able to sense it though the metal conduit well but if you're lucky you can find a plastic section in concrete near the wall where it stubs out or some such. If you fish out of the metal pipe and into a void you'll also be able to detect it there.
By now it may or may not have also been vacuum or air bottle time, again depending on how far the job box is that has what you need. This works better on sealed types of pipe, but can even work with EMT and screw couplings. The more powerful a vacuum you have, the better it will work. The first difficulty is attaching the vacuum to the mouth of the pipe so that all of the suction is applied to the pipe. With an air bottle an assortment of rubber nozzles are available designed to get to knockouts in the back of a box, but for a vacuum, you may wish to use duct seal and duct tape to seal off the rest of the box and attach the vacuum to the mouth if you don't have a suitable hose to seal well to the knockout. Turn the vacuum on before you attach it and note how much it labors when you attach it. Lower quality vacuums can be burned out this way. If the vacuum is working extremely hard I'll use it 30 minutes at a time max, and I may be able to conclude that the other end of the pipe is completely blocked, which is valuable information. Once I have the vacuum set up and air is flowing I can search for the other end by listening and feeling for air movement and or using smoke in likely areas to see if it reveals something. If the pipe happens to be core line, it will also often shreik like a banshee which can be revealing and very irritating. If the pipe is screw coupling EMT, you lose some pressure at every coupling, so you may have to be very sensitive of the suction that remains at the other end.
Be ready for surprises when using a vacuum or air bottle. An abandoned pipe could be full of water which will blast everywhere if you use an air bottle and could indicate the reason the pipe was abandoned. This usually happens when a plastic pipe gets filled somehow during construction, so a pipe that was sealed and has had years to evaporate is little concern.
Maybe we'd like to try a more extreme option and risk the pipe since we've run out the other options. If it's a type of pipe that is reasonably air sealed and mechanically solid, like pipe run in concrete, continuous plastic pipe or rain tight EMT and you think it comes out inside a void, You can place a Marette or ball bearing (Smaller than the pipe or it could jam and note it could jam anyway) in the pipe and use an air bottle to fire it as a projectile. You want listeners everywhere as you may only get one try. The most extreme version of this I've seen is an outside company brought in to fire a "pig" which is a large metal version of a cleanout with two full rubber seals down a larger run of PVC. Comes out the other end with enough force to smash a wall, so it was done to clean out and fish a difficult 4" pipe before walls were put up. If the pig gets stuck you can put the air compressor on the other end to free it and try again, but if that doesn't work, the pipe and the pig are lost.
If I think of something else I'll add it, but this should at least get you putting information together in the right way to solve your mystery pipes.