I have a stand up shower in the upstairs bathroom. I am now experiencing staining on the bathroom ceiling beneath the upstairs bathroom. I know its not a supply leak because we held off showering upstairs for months. I removed all the shower caulking and replaced it, thinking water was getting past where the plastic wall meets the floor "tub". It was fine for a couple of weeks and now its back again. Could it be a leaky drain? How do I access the drain plumbing without destroying my ceiling? Or could my first instincts be right and I am not using the right sealer. Everything is PVC on the drainage since the bathroom is only about 8-10 years old.
Thanks
Sniper

If the floor gives a bit when you step in the shower then the drain seal may have ruptured. I have put up a shower drain,(see image) to show you what you have under the floor. To test for a ruptured drain seal block off the drain and fill with a inch or so of water and watch the ceiling. If you see fresh moisture then drain the water out. You have a bad seal. You may get lucky and run some silicon jel around the lip, pushing it under the lip as far as you can and letting it set up but to do it the right way the ceiling will have to be opened up and the large nut loosened so the lip can raise up a ways. The drain installs like this. First the drain and the putty seal under the lip. That's on top. Under the shower floor you have the rubber gasket followed by the fiber anti-friction washer and finally the nut.
Take and make a rope of plumbers putty and force it under the lip. Tighten the nut and you have a new seal.
However, the problem will be back unless you can bed the floor of the shower with mortor, cement or support it somehow before you patch the ceiling.
Good luck, Tom