We were having a frequent drip show up on our stairs from the master shower above. The tiled floor of the shower (over concrete pan) had mildew in the corner, so I cleaned out the corner and found water between the tile and shower pan membrane. So, I pulled out the lower rows of tile and water came out. I also took up the showers tile floor. We've been just ignoring the shower fix for awhile, but today, I realize that if I hold a bucket under the shower and run it, a dark damp patch appears and grows on the concrete. I also opened a hole in the wall of the bedroom and the vertical pipes are not leaking. Did I mention that I poured water down the drain to see if that caused a leak. It didn't. SO what is happening? It's obviously with a pipe, but what pipe? Do I have to take out this concrete shower pan? Thanks.
I think the fact that a "dark damp patch appears on the concrete" when you open the shower valve(s) yet catch the water in a bucket points directly to a leak from the plumbing. You need to open the wall from behind to gain a view of the shower valve body, and also remove all trim on the inside. Use the handles to open the valves and find the leak source. Water may be running down the interior of the enclosure wall. The fact that you have a plumbing leak does not mean that your shower pan is in good shape; if it is old, since you have removed the tile, this is an appropriate time to break out the concrete and have the pan refurbished (hot-mop, vinyl liner, fiberglass pan, etc...)
I agree with Jimmy Fix-it. Until the leak is remedied, I'd just caulk under the valve to seal water out from getting between the tile & wallboard. Your bigger problem is why wasn't the water draining under the tile or pan & simply finding its own way out.
Consider yourself lucky. You need to rip out the pan. First, water shouldn't find any way out. And Second, your pan's membrane should be emptying into your drain's secondary inlet that the membrane is supposed to be applied up to & is there for this exact purpose.