I know issues of sewer gas smells have been addressed repeatedly on this forum. I have used much of the advice here to try and figure out where our sewer gas smell is coming from. Last week, I gave up and had a plumber come in to try to diagnose it. He left without telling me anything, although he did locate what appears to be an odd "drain" that has been capped off UNDER the basement bathtub. He said "hmmm, maybe that's leaking". A little back story:
The house was built in 1980, the last house on the street at the time and remained that way for more than 10 years. It is a tri level house. My husband purchased the house in 1990 with an unfinished basement that had the plumbing stubbed in for a full bath. He hired people to build the walls and set the fixtures to create a full bath down there. He is unsure when the smell started, but is fairly certain it was within a few years of purchasing the home, if not immediately. We have no sewer smell in the cooler months, but when the temperature gets above about 65 degrees, we can count on having it at least once a week, if not more. All of the plumbing, with the exception of the kitchen, runs down the center of the house, dropping pretty much straight down from one floor to the next. Bathroom on top floor, laundry room just below that and basement below that. It all "ends" in the basement closet that is behind the basement bathroom. When the smell does come, it is strongest in the laundry room and the basement closet (although it permeates the entire house at times). We have never smelled it in the upstairs bathroom, and it is never strong in the basement bath. We have smoke tested the vents and got smoke out the roof stack(s). We do, on occasion, have a slow drain problem with the basement bathtub, but this is a recent development and never coincides with the smell. I noticed there is an "s" in the plumbing configuration not too far before it hits the vent. I was thinking it should maybe be a p trap there, but the plumber said no, it shouldn't make any difference. I thought maybe the water was siphoning out of the "s" and leaving us with a dry spot there. We've traced the vent up through the attic and it is all intact. We've been on the roof and the stack isn't blocked. We are on public sewer, not private septic.

So, first, any ideas at to what that mysterious capped off drain under the bathtub is? Should be noted there are no exposed floor drains anywhere in this basement. Then there's the 64,000.00 question: WHAT is causing the sewer gas smell? The plumber I had here could only give me "maybe it's this, or maybe it's that". We really don't have the budget to have someone in here on a "maybe". Any help at all is greatly appreciated.