I was having the same smell in this rental I moved into last March. Just so happens, in one of the Spring issues of a magazine-a professional plumber had an answer to this problem. He said often the sewer gas will not escape & I did what he suggested-and I still do it 2x a month. NO more odor! In every drain in your home..kitchen, bath, laundry...run hot water in the sinks & tubs for 1 minute. Flush all toilets. (Are you ready for this?) Measure 2 TBS of vegetable oil & dump it down all of the drains & let it set for 5 minutes or so before running water anywhere. I swear-the smell goes away within the hour. It creates (-for lack of a better word) a..skim covering over the drain holes & prevents the gases from coming up from the drain. Cheap way to solve the issue without a plumber. try it-can't hurt.

Do the bath vent fans share duct work? Does your have a damper that closes when not in use?

Thanks maryland irisman. You're on the same trail I'm on. I strongly suspect we're getting a back-draft effect down through the exhaust vent. When I disassembled the exhaust fan housing a couple weeks ago I noticed the "el-cheapo" plastic flapper didn't seem to be closing properly. That's what led me to seal off this exhaust vent. Also, when I went up on the roof to inspect the vents for bird nests, etc., I did notice the odor right at the eave just below where the plumbing vent exits the roof. This plumbing vent is about 2 feet further up the roof from my exhaust vent....so if the lower evening temp could be allowing sewer gas to drop down toward the exhaust vent housing maybe that's where the odor is getting into the house. I am about ready to start unsealing my drains, overflows, etc. Once I am able to determine the exhaust vent is the culprit I will experiment with raising the plumbing vent stack. Thanks for your help, and I'll post the results.

Thank you David! When having this house renovated post purchase, much of the bathroom was gutted and the sink I had installed is one I bought new some years before and was waiting to use (American Standard Standard Collection Console sink with oval glass shelf.) Because the works underneath are totally exposed I bought the best chrome P-trap and fittings a local plumbing supply house had. In a month or so I'll have my plumber back to install a pump for my utility sink, and he will need to add another vent stack I believe, so I'll have him see if the sink is connected to the bath vent, and if the vent has any blockage. I'll ask him about the air admittance valve - thank you so much for that information!