OK need help. After the ceiling on my the first floor became wet and sagged I cut out the 2x3 foot opening to examine(luckily was in a closet) which is directly underneath my upstairs shower/trap.
The Shower and fixtures I believe are original about 20 years old. The Shower is a one piece unnit and have one of those single acrylic ball like knobs that you lift and turn for water control/temp.

So here's what I've done so far:
I dried off the moisture from the pipes/area underneath and placed basin underneath.
-I ran the water through the lower bath spout for a few minutes, No leaks
-Plugged+Filled tub as high as I could without reaching the overflow. Nothing leaked after 20 minutes.
-Drained tub again, nothing leaked.
-Ran shower head for about 10 minutes. Nothing leaked.
-Took a shower with me standing in tub and caught about 1/4cub water in basin. Visually from underneath it appears a small stream was running from the inside fiberglass shower wall down onto the pipes/trap then into my basin.
-the next day I tried the same thing with similar results.
-I next tried taping a large sheet of plastic high on the shower head wall covering all the fixtures and spouts to see if water was penitrating through any of the fixtures due to bad caulk etc while showering. Still got roughly the same amount of water in my basin.
-Aren't any soft spots in the pan that I can tell or notice when standing in the shower. (and I run about 230lbs).
-One thing maybe is when showering I do adjust the control knob a lot to control the water temp. Would this control knob leak from the backside if turned a lot during showering?
Running out of things to check. Does any of these symptoms sound familiar to any common problem?

Try one more experiment, go back to step #3 where you filled the tub, now get in and stand in the tub to see if it leaks. I think it may due to your weight standing in a bath or a shower. The acrylic tubs like yours can flex and that flex over years can cause the drain to come apart just a hair with your weight on it.

small stream was running from the inside fiberglass shower wall down onto the pipes/trap then into my basin

Does that mean the wall with the shower head and valve?

If so, pull the escutcheon around the shower arm away from the wall, with a flashlight, look inside the wall to see if there is any sign of water.

Better yet. If the shower head screws on to the shower arm, get a 1/2" threaded pipe cap. Cap can be PVC or steel. Remove the shower head and install the cap on the shower arm. Turn on shower. If there is a leak where the shower arm screws into the drop ell, having the shower arm capped off will put full pressure on the connection and any leak should show up.

You mite try drying the tub out with heat gun. Not too close you will melt the shower insert. Caulk inside of shower and let dry. Check next day to make sure no caulk has been pushed out by water. Also you could have a gasket that could be leaking on the main handle. If its leaking just when you stand in the tub. You mite need to remove the drain flange and put new plumbers putty on the drain flange. You buy a tool to tighten the flange tight. Good luck. Hope that hepls.

HI Giggerjoe...

Did you just tell someone to take a HEAT GUN and use it on a 1 piece fiberglass tub that has nothing to caulk? I have to say, that in my opinion, that is not super good advice here... ;)

I will, however, agree that the tub shoe strainer could be the issue, but if that is the case I would recommend reinstalling the tub strainer using a flexible silicone caulking as this will help to reduce the chances of this being an issue down the road if the tub floor actually does flex.

Helpne1, please try the things mentioned by Ballenger and Hkstroud, and let us know what happens. It could be any of the above, OK?

Thanks...

Mark

I will try all of the above minus the Heatgun thing. (Maybe a pro could use one without damage though I'm not that familiar the temps involved.) Will post my findings this weekend. Thanks for all the suppoer so far...