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.
I used a thermographic camera to detect pipes in my house. Let the hot or cold water run for 2-3 minuttes, and the pipes heats/cools the wall very slightly. This is very visible on even the cheapest thermographic cameras.
I know this is an old question, but 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.