Typically there is a pan under the showerstall. The pan is uasually a couple of
inches high at the walls and the walls go inside the pan. The pan is supposed
to catch any water that gets by the tile and grout and send it to the drain.
It's really not that difficult to find the leak. What you need ot do is get a
mirror that can be aimed in different directions and a good flashlight. Hold
the mirror up thru the hole in your plywood while someone is using the shower.
You should be able to tell if it is coming from the pan or form the piping that
is in the wall to feed the shower control.