I know metal detectors for walls, but in my house there are places where it has a plastic pipe. I wonder if there is a good detector to buy.
Typically in the ground they use expensive sensors and ground-penetrating radar to accomplish this. If you can find a stud detector that can detect water in the pipes (you need something to conduct), then you should be in business.
This company makes a plastic pipe detector. I imagine it uses radar to do it or sound waves.
This is a common issue: find a water plastic pipe into the wall, either to work on the pipe or to avoid punching it.
In my case I was able to locate such pipes more than once using the sound of the water running through them as a guide. there are many ways to do that.
Just let the water run through the pipe and use a big, long screwdriver as a sound probe. Firmly press the tip of the screwdriver into the wall, and also firmly press the handle against your ear, best results are achieved if the tool is kept orthogonal to your head. Just try it and you will discover what position gives you the best sensitivity. Now just probe the wall in different places until you locate the pipe. Another option is to use a microphone and an audio amp, or better yet a stethoscope.
It's worth mentioning that there are very inexpensive borescope or "snake" cameras which attach to a mobile phone or computer by usb, and could easily be inserted into a wall or other area to see what's inside.
These have the advantage that you can obviously directly see what is inside, including other things you were not looking for that could be relevant, especially blockages etc.
In many cases you might be able to take advantage of some existing opening in a wall (such as a hole on the inside of an electrical box) but if you need to make a hole they are pretty small.
A side advantage is that these cameras are useful for all sorts of things, one task can justify the modest cost and then you have it for other things.
That said, if you have to do a very broad search this is probably not the most efficient approach.
The Bosch Digital Multi-Scanner GMS120 claims it can detect plastic pipe that is filled with water, so probably doesn't work for detecting drain pipes or vents. I have not used it and it has mixed reviews on Home Depot and Amazon.
If trying to mount something to a wall that has dry wall, best thing I've found is to use EZ Anchor corkscrew style drywall anchors, but instead of of drilling a hole or using the anchor to drill into the drywall (like it is designed for), I use a #2 Philips screw driver, and spin back and forth as I slowly press it through the drywall. If there is a pipe or a wire behind the drywall you will feel the resistance against the screw driver once you get through the drywall and the screw driver wont puncture the pipe or wire like drilling would. IF there is something there, patch the small hole and move to a different location, if not, you have the perfect size hole for the end of the EZ anchor to slide into and then finish installing the anchor by screwing it into the drywall the rest of the way.