Our house is 5 years old and our shower stall is leaking. We noticed our baseboard is swollen and when we took it off there was bad water damage to the sheetrock. It was covered in black mold and water was in the area were the baseboard and molding use to be. We have tried everything. The plumber said it is a tile and grout problem and the tile guy said it is a plumber problem. We have plugged up the drain and within 10 minutes we can see water leaking out into the baseboard area. So, does this prove it isn't the tile and the grout, because we waited overnight and the water in the bottom of the stall was at the same level. The plumber took off the showerhandles and there was no signs of water damage on that board. My number 1 question is how can the liner of the shower get filled with water? Also, when the plumber was here and he filled up the bottom of the shower stall I walked in the shower stall and the weight of my body called the water to "gush out" into the floor. There was that much water in the pan of the shower. So, does it sound like it is a leak in the pipe, or is it the pan. Again, how does water even get in the pan?
Help!!

My number 1 question is how can the liner of the shower get filled with water? Again, how does water even get in the pan?
If I had to lay blame I would lay it on the tile man. Let me explain. Water will seep between the tile grout and down through the mortar when the shower's in use. This is why weep holes are in the base of your flange type shower drain,(see image). The weep holes allow the water that seeps down to drain back into the trap. If the tile man allows mortar to get in the weep holes and seal them off any water that seeps down will build up and leak out. Hope this helps, Tom