Re: Leak around shower drain
If the walls and floor are sound, then it could be leaking around the shower valve/head penetrations or around the door/shower curtain. Is this a one piece fiberglass unit or is it a two or more piece? The seem where sections come together can leak without showing on the surface. Caulk usually will take care of this, however if the joint is wet when latex caulk is installed, the caulk won't cure properly and will wash out.
You said the plumber caulked around the drain - that's not a suitable repair for a drain leak! There are two styles of drain that I know of. One uses a rubber donut that is forced in between the drain pipe and the drain neck on the pan. The other is a threaded "basket" that uses rubber washers to seal itself to the pan and drain. The former is pretty easy to fix, remove the existing donut, clean the mating surfaces of the pan and drain pipe, install a new donut. The latter style of drain isn't what I would call a DIY project.