On Friday, I spoke to a warranty guy from the builder about the issue and how we were blown off by the plumbers both times after a negative smoke test. This is not the warranty guy who came out a couple weeks ago to do all our "one year" fixes. This is a different guy we really like, who was incredibly helpful when we had a major leak in our fridge last year. The water valve in our fridge, which was included with the house, started to leak whenever the ice maker was filling up and slowly dripped water onto, and under our vinyl floor planks for what must have been several weeks. We had to have a base cabinet completely replaced, and had 2/3 of the floor planks in our kitchen replaced, along with some drywall, baseboard and shoe molding.
Anyway, this guy is a solid dude, so I reached out to him directly about our frustration. He's coming out in a couple weeks to try some other options. Our laundry room is on the other side of the shower wall, so he's going to remove the baseboard, then poke a small hole in the drywall and snake a camera in there to look around under the shower and at the pan itself to see if we can find standing water or water damage. If we find something bad, then GOOD, we'll at least have the culprit. If he finds nothing, then I guess it's on to other options.
I don't know about the quality of the p-trap construction, but while the warranty guy is here with his camera, I'll have him look down into the trap and see if anything looks amiss.
As far as plugging the shower and steaming up the bathroom, I doubt that would find anything helpful. My wife and I both take hot showers, and this smell has come and gone since last summer. We've had the smell come up during hot weather, cold weather, high humidity, low humidity, bathroom door open, bathroom door closed, bathroom fan on, bathroom fan off, etc. That's what makes this problem so maddening. There is NO pattern to it whatsoever. In fact, we haven't smelled the odor for 2 or 3 days now, and it may stay gone for a week or two before coming back again. This is the most random and elusive stink ever.
Our bath towels are not making the smell. My wife and I are both neat freaks and our towels are replaced with clean ones like every 3-4 days. We clean the house every week and my wife scrubs the shower and mops the floors herself. We clean no differently now than we did back before the smell began either. Same cleaning chemicals we've used for years. Our master bath is quite large, so not a small confined space like you'd find in an apartment bathroom for example. Harder for things to get really steamed up. The smell is not coming from something in the room. Again, when the smell is present, it ONLY comes from the shower drain itself. Not a towel, not the drywall, not the grout, not the silicone sealant on the shower door. ONLY the drain. Since the trap is wet, then that logically means it's coming from under the pan itself, or somewhere within the walls, and then making its way to the shower drain opening via underneath the pan.
I'll report back in a couple weeks after we've had the camera stuck into the wall. Hopefully will have some good news by then.