nicktheplumber
There is one problem with the metal-detector solution, besides the situation where the pipes are non-metallic, and that is that many slabs contain metal wire/rebar, which confuses the detectors.
There is equipment that will detect the sound of water flowing through pipes. There is also a radar that can be used to get an image of what’s under the slab. There are specialized contractors who have this very expensive equipment, and I’ve seen both types used to locate hidden plumbing and rebar.
