I moved into this condo for 3 months and I realized a part of the wall in the master bathroom has a leak. It‘s near the corner of the bathtub. I suspect it‘s the pipes leaking behind the wall. The contractors said it‘s the water escaping from the shower curtain. I am still not convinced because I didn‘t turn the shower on for days and then the wall dried up. I turned it on and let it run on it‘s own and I saw water coming out of the wall. The contractor said it‘s pressured copper pipes and it‘s impossible for it to leak. If it did, the whole wall would fall off.
The leak is past the bathtub/shower valve where it is only pressurized when running water to your tub. I had a similar situation a couple of years ago. In my case it was the valve itself which was leaking. It also could be the drain connection in your tub. The low cost approach could be to have a plumber (not your last contractor, as he should have known this) come in and repair the valve and possibly drain connections. If there is an access panel on the other side of tub wall, you should be able to determine exactly where the leak is; it also may not be too difficult to replace the entire tub/shower piping. My problem was pre-existing when we bought the home. Not knowing the extent of the damage and not having an access panel, I had to remove the entire tub surround and wall behind the tub faucet. I replaced the faucet valve and shower piping above it. I also replaced a section of plywood flooring and part of the wood studs that were completely rotten and also areas with some mold. If your problem has just started recently, you shouldn't have to worry about all of this, but you need to get it fixed before it does become a major problem.
Of course the pipes could leak. The connections often leak if not soldered correctly. It could also be the plastic drain piping. One of the joints, probably on one of the 90 degreee elbows to the shower head would only leak when you ran the shower. Did the contractor pressure test the piping before he put up the drywall? Can he prove the pressure test was successful? Your contractor needs to investigate. But of course that would cost him money to cut a hole in the drywall, repair the leak, and then repair the drywall. It's much cheaper for him to convince you the pipes could not possibly leak. A reputable contractor would have already fixed it. An easy check you can do yourself would be to take off the shower head, screw on a 1/2 threaded pipe cap, and turn on the shower. If the leak appears you know it's in the copper piping behind the wall. The photo helped. Either there is a leak between the tiles or in the piping behind the wall.
I had this exact problem once, it turned out to be the pipe from the tub spigot to the shower head. It only leaked when the shower itself was turned on, not the tub. If you have access to the wall behind the tubset, cut a small hole, turn on the shower and look through the hole with a mirror and flashlight. Look both up and down, the leak could come from either end.
