I have a fiberglass bottom shower with marble sheets for walls and a glass sliding doors. I have noticed the linoleum is disclored and soggy underneath but only in the corners near the front of the shower were you step in. I have recaulked the whole shower with a good silicone and also in the metal frame corners that hold the doors. I am pretty sure water is not coming from inside the shower unless the fiberglass has a crack I cant see. There is no staining of the ceiling in the downstairs closet which is under the shower. Water must be moving laterally from drain area in the subfloor. I have a metal strainer that snaps off the drain and I see a piece that screws into the bottom part of the drain assembly that needs a special tool to unscrew. It looks like there is a gasket there and it looks compressed and old. Could this be the source??

speedball1

Hey Jeb,
I don't see where you have much choice but to open up the ceiling and find out exactly where the leak's coming from. Until you've done that you're just guessing. Most leaks in manufactured shower floor come from a faulty seal around the lip of the drain or "spider cracks" that develops in the floor itself.
Regards, Tom