I've seen you giving some very logical advice to people in terms of so many different things that I won't bother to go into the most obvious places to begin as I am sure you have already done the obvious...
Fist, I am wondering if the leak shows up only when a fixture is used? Or is there a constant leak and you shut the water off for now? Can you isolate this to be a heat pipe, a cold water pipe or a hot water pipe, or could even be a waste pipe if leak only occurs during use of a fixture?
If not sure, shut off the hot water heater and see if the leak stops. If you can isolate just the hot and/or just the cold water to this bath then do that... or is the leak so large that when you turn the water back on it just starts making a huge mess..? If heat pipe, then may make it easire to isolate, but you need to tell me more here.
You could also use an air compressor
to pump pressure into system and see if you can HEAR anything obvious! Then, I guess I always start at the easy stuff
. Remove the tub/shower trim plate and see if you can see anything obvious behind the plate. Next, if there is an access panel behind the tub/shower open that up and see if there is anything happening there. Of course, look under the vanity and check that water is not streaming down the water pipes and showing up downstairs.
After that, I usually decide to open an area of the ceiling. They sell spring loaded access panels
at all home improvement stores so you can plug the hole cleanly until ceiling can be patched.
Finally, if I have to open a wall I usually open a hole in the wall that can be covered by a picture
until the plasterer can get in to patch the wall. You can also remove some baseboard, cut the wall behind the baseboard and see if you can find a pipe that water is flowing down... then follow that pipe upwards.
I will say that with good detective work you can usually find these leaks without too much damage to the house...
Answer some of my questions... see what else we can come up with.