The Key to finding this leak is as follows

1. Put a stopper in the shower waste ( normally a 2" plug) GET 2 or 3 FIVE gallon pails of water FROM another place OTHER THAN THE SHOWER.FILL the pan to about 3" high and let it stand over night If the water holds with no leaks your 50% HOME free.

2. NEXT let the water down the drain AFTER the 24 HR waiting period and pour in SEVERAL more pails of water AGAIN getting this water from another sourse.

3. NOW here comes another BIG TEST. CAP all your shower heads and TURN on the shower body for about 10-15 minutes ( DONT USE any, water in your home as this test is being performed as you dont want to have a cross /mixing of H&C water throughout your home. Again if NO leaks you are in fantastic shape as you now know YOUR shower pan and shower body are in great shape.
4. Now take the caps off the shower head ( if more than one leave all but one capped). When you have uncapped the shower head put on an adapter HOSE x Female adapter. If you have the newer type of shower head (less ball) attach a washing machine hose and direct the water all around the outside of the shower body. We are looking for leaks around the escutchions (floor gate). If after 5 minutes no leaks THEN direct the water to all the walls and this will test the water tight intergrity of the grouting.

Then if this proves ok get into the shower and close the door and aim the water at the shower door to see if any water spashes out of the shower.

One of the following above should pin point the problem. IF all else fails try putting the hose down the drain a few inches to check for a possible separation of the existing piping.