I had a similar problem in my old house in SC.
The master bath had a shower surround with the classic shower base, not a cultured marble floor, but after a couple of years I noticed a weird squishing noise when I stepped in and out of the shower.
We (ummm, to be honest that should read HE, as in my hubby) went into the crawl space and looked everywhere for water leaks...nothing...nada.
So I let it go for another 6 months, till the squishy noise just wouldn't quit and I was getting nervous about the entire floor falling out one day.
I wanted to remodel that bathroom anyways, so I got busy and started tearing stuff out. Had to remove the walls first to get to the flooring. I yanked up the base and found a mass of rotted subfloor and joists.
At that point I called in an expert and asked him to repair, replace, etc.
He was in that bathroom for about 20 minutes before he called me in to show me the problem. There were two of them actually. The drain pipe was no longer attached to the drain! It had worked itself loose and allowed water to shove itself back up under the base occasionally...not all the time, guess it depended on who was in the shower and how they moved around on the floor.
The second problem was a slow leak from the hot water pipes. It had been slowly dripping down the pipe and back wall...then onto the subflooring, where it pooled and most likely dried itself up before leaking clean through the flooring to the crawlspace.
The plumber explained it this way: There was leaking, but unless we took several showers in a day, there wasn't quite enough water to seep through the flooring, just enough water collected to make the squishy noise. The water then slowly evaporated...till the next shower.
He pointed out staining that we couldn't see when we inspected the crawlspace. It was there, just not moist and glaring at us.
Ended up tearing it down, replacing a ton of wood, including two rotting joists and then redoing the plumbing and putting in a new shower.
Whatever you do, don't ignore it or try the "let it dry and reseal" technique for the floor. It might mask the problem for awhile, but it won't cure it and you'll just end up spending huge bucks later on when the floor eventually rots out completely. Better to tear it all out and fix the issue and then replace with a new shower now.
Strawberry
PS You just might end up with a shower you like better too...I did! I found a false wall when I was tearing things out and ended up with a full walk-in shower instead of a standard 36" stall shower.
Cost of the new plumbing, new joists, new subflooring and new backer board: $1500.00
Cost of the walk-in shower and new ceramic floor to match it: $1500.00
Cost of new paint and new lighting: $200.00
Cost of a new wax ring to replace the toilet when the new flooring was in: $5.00
Peace of mind when the squishing stopped: Priceless