I am also in the camp that you have a leak.
I've lived in a couple houses with a leak in a shower. The first was a failed/improperly installed shower pan which necessitated pretty much replacing the shower. The second someone had hung the drywall from the copper shower head riser pipe, one the screw corroded away enough every time the shower was used it would leak out the hole.
Basically, my take is you have a leak either from the plumbing or the shower itself. Likely the only way to tell/fix it is to open the wall. Plus if you have water you probably have mold, if not rot, that you might want do address depending on your sensitivities.