no offense meant to you; i will say that your descriptions are too vague for me to follow meaningfully. Where you CAN see, what you can see, etc. I'd prefer knowing precisely about studs and joists, vapor barrier, back side of the tiled wall, etc.
Without knowing more, I'd guess now that you have water dripping down the back side of the tiled wall, inside the wall. If it starts right at the plumbing, you probably won't see it from inside the shower. You can probe and test with a kleenex on a stick to it to see if the tissue comes out wet or damp.