Update for anyone curious: The builder rep stopped by with a moisture meter on Wednesday and determine that the baseboard was in fact damaged by water and still was retaining significant moisture.
He called a remediation company who arrive early the next day, and start their analysis. They turn on the shower and used a gadget to try to detect the water leak through the wall but determined there was none.
The builder rep arrives a short while later and the techs explain their findings. The builder rep then insisted they remove a portion of the wall to be absolutely positive. I observed the pipes in operation and at rest and can confirm that the pipes are not leaking:
They also removed the damaged baseboard and cut a chunk off the dryawall to expose the frame of the house. The shower has been running continuously for about an hour at this point, but even with the baseboard removed, there does not appear to be an active leak.
The techs did measure a significant amount of moisture in the wall:
So the remediation company theorized that there was probably some sort of water leaking outside the shower, onto the floor, and it was getting soaked up by the baseboard and/or there is some kind of hole in the caulking allowing water to get behind the marble.
They based this theory on the fact that the baseboard was observed as being more wet on the front vs. the back of the baseboard which was less damaged/less wet. Apologies I forgot to take a picture if the baseboard. However, I did observe it myself and I agree, it generally looked wetter from the front vs the back.
The techs then cut a gap in the drywall where the baseboard once was, remove some of the caulking in the shower, and loosen up another baseboard in a closet. They then place a large, very loud fan and aim it at the exposed wall frame:
At this point I am not fully convinced of the theory that the shower door or caulk is allowing water to leak water inside the wall and outside the shower. After all, my wife and I take great care to squeegee water off all the shower walls and door every time we shower- how could water be escaping when we open the door?
So I insist they call a plumber to come out and take a look, and he arrives early today (Friday) in the afternoon. He inspects the stall and of course immediately notices the missing caulk, which I explain was removed the day before by the remediation techs. He continues to examine the shower, looks at the piping, the fixtures, and the drain.
Then after a little while, he provides basically the same explanation for the water damage as the remediation techs, that water is escaping through gaps in the caulk and or shower, falling outside on the floor, and getting soaked up by the baseboard. He explains if it was the drain then other surrounding baseboards would show signs of water damage, which they do not.
I ask him how he came to that conclusion. He asks to borrow a cup, then fills it with water. He gets inside the shower and shuts the door. He then throws the water from the cup into the corner of the shower where the shower door hinge is, from a position similar to where I am standing in the pic below:
When the water hits the corner of the shower, to my surprise and amazement, a decent amount of water splashes outside the shower and beads up all over the exterior marble and floor- even while the shower door is completely shut.
The plumber then explains that water is probably more that could be done to seal up the shower door, and he says it will be up to the builder to figure that out. He says he will call the builder and report his findings, and I thank him before he leaves.
So I am starting to think that the water damage could be in fact due to loose caulking and gaps in the shower door. I am definitely happy that the wall was opened up and it is confirmed the pipes are not leaking.
However, there is still a small voice inside of me that still suspects the drain clog for the leak. Iām even thinking about testing it myself tomorrow using this method:
Am also going to figure out how to pop the strainer and inspect the gasket like Geofd suggested earlier in this thread.
The remediation company is coming back Monday to recheck the moisture. Assuming the moisture goal is reached, then the builder is already planning to come out next Wednsday to fix up the wall, carpet, basboards, and re-caulk the shower. I need to talk to the rep to see if they can do anything about making the seals around the shower door tighter as well.
Anyhow, thatās whatās going on. Didnāt want to leave you all hanging because I know youāre all losing sleep worrying about it like me haha j/k. Once again, appreciate all the help and suggestions from the replies in this thread.