I have 5 circuits that I cannot identify on my breaker box. I have included every known end-point including furnaces, outdoor outlets, garage, etc. I was wondering if there was any way to disconnect the breaker in question and use a signal source at that end and some type of signal sensor to walk around the house looking for the signal. I do know in which direction the cables exit the breaker box; to the crawl space or up to the ceiling/second floor.
"Breaker Finder" (mentioned above) or "Circuit Breaker Detector" is a must-have tool as well as a label maker IMO. I bought an inexpensive detector from Harbor Freight for under $20 thinking that it was a one-time use. Two houses and countless labels later, it's still there when I need it. Also, you might want to grab a lightbulb socket to outlet adapter for ~$1.00 and use the detector's receiver in that.
Another suggestion is to see if there are multiple wires into the back of a breaker. It's an illegal practice and potential safety hazard (although some people may debate this.) It can screw up readings from breaker detectors as well as leave you confused why other areas of you home might are affected.
Also, panels are not allowed to have completely empty slots (holes.) instead of a metal slot cover, someone might've stuck an unused breaker in there to fill in a void.
Last but not least, someone might've left a breaker in but terminated/capped off the wiring. If so, hopefully it's at least in an accessible junction box.
When I have this happen, I normally recommend to disconnect the feeds (hot, neutral, and even ground) inside the panel, then cap and label them. Include a date. If you don't have a labelmaker (everyone should have one of these!) you can use the plastic sheathing from Romex 14/2 or 12/2 and write it on there. Then slide the sheathing over the wires, and tuck them neatly behind the rest. Do not cut them shorter, as there is a chance you'll stumble across what they were for down the line.
If it's labeled in some logical, coherent manner, anyone in the future (including yourself) will much appreciate the investment of your time so they don't have to do the entire process all over again. Houses change hands many times over their lifetime, and the little things like writing the circuit number on the inside of receptacle cover plates makes an average electrician suddenly look like Nostradamus.
There are some options to try
1. Visual Test. A simple visual "make and break" of the black hot wire while the breaker is on to see any arcing will indicate a load. No arching indicates no load, but could also mean an outlet with nothing plugged in, or a light bulb that is not on. But definitely it is not a always on load like a refrigerator.
2. Use a AC circuit breaker finder. I can say from experience this is a pretty handy little guy here. Takes a little practice though!
4. Leave off and see what is not working. This is usually the best option I think.