joecaption

If it was mine I would remove the shower spout and insert a nipple with a cap on it with teflon tape on the threads, open up the valve so the shower would be activated and wait 24 hours, then look for a leak. I use a piece of toilet paper and wipe the pipes. If there is none then it can be as simple as the shower spout (the curved chrome piece) was leaking. If it is leaking then you have a bad soldering job on the drop elbow ( what the spout screws into and will need to fix that.

isola96

Well that answers why the spot was on the wall good thing you opened it up.

nealtw

There is a corner bead in the picture, to me that says, this was an archway or closet door at one time. If the water came from the toilet upstairs the water could have travelled some distince behind the cornerbead. You may want to cut another hole above that framing and let it dryout. It won't dry out if it is covered with plastic.