I have a problem with my shower leaking as well but mine is slightly different. When my shower is running it will leak but the water shows up on the basement floor. The shower is on the second floor. I have moved my drop ceiling and found that the back wall is wet along with the joist along the outside wall. I can't determine where the leak is and there are no signs of water damage on the first floor. I am stumped by this and afraid that it is going to cost a fortune to repair unless it is something I am overlooking.