This is a corner shower. Forgive me, I don't know the correct terminology to use, but when the contractor installed the "pan", he did not first attach the "drain" to the PVC, so my dad used concrete to make a "funnel" from the pan into the PVC.
It's been several years since it was installed, and my daughter uses the shower. I've noticed the grout on the tile floor is sometimes damp, the subfloor has water marks on it, and the trim around the door (which is right by the shower wall) on the very bottom is soft and mushy.

Thinking it was cracked plastic walls, I replaced them with tile -- the plastic shower wall was cracked around the shower faucet handle-- Figured this was the problem with the leaking, but looks like I was wrong.

Called the plumber, he said it's not the pipes. He said it could be a bad tile job (well, thank you very much), missing grout, the shower pan or the drain.

Before I take a pry bar and chisel and take off the tile, is there any way to test where the leak is coming from?

IE:

If I stop up the drain, leave in an inch of water and it is no longer there in the morning, does that mean it's the pan?

Do I cover the tile with a plastic shower liner, down to the pan and see if the seepage is coming from the tiled?

Any help will be so appreciated!

What kind of "pan" was installed initially? I would guess it is the joint between the pan and the drain or possible the pan itself.

This is a concrete floor, this is a pre-fab one piece plastic with the drain hole. I can tell you it was not an expensive shower to start with, and the plastic did seem flimsy.

Is there any way to confirm what the problem is?

Those pans are supposed to be fitted with a flange that screws through the pan and then presses through an adapter on the drain pipe. I'm almost sure the flexing of the pan has caused the cement to break away from the pan.

Those pans are supposed to be fitted with a flange that screws through the pan and then presses through an adapter on the drain pipe. I'm almost sure the flexing of the pan has caused the cement to break away from the pan.

It seems that the water damage is localized to that one small section -- I'm not sure about that, but it appears to be the case. I guess the only way to know for sure is to pull the pan, the first row of tile and see where the damage is. I was never happy about the missing flange/adapter. Thank you for your help!