Water has a mind all of its own, it gets to places and leaves us to wonder how. I guess all you could do is a process of eleimination. Try not to use the shower for a while and see if it dries up. if it does dry up then run the shower directly down the drain so there is no chance of it going down curtains walls etc. If all is still dry then the leak is finding a gap and running down the wall as opossed to a leaking pipe inside the wall.
steve
November 2004