We have a sewer gas smell in one of our lower bathrooms around 1 to 2 times a week. The smell only last for a couple of minutes, then it is gone. You can smell everything in the bathroom (toilet, air vent, shower) and it is simply in the air and nowhere else. When the smell happens, it is always at night when there is activity in the bathroom. The house is only one year old. The plumber has done the smoke test, replaced the toilet seal and removed the cap to the PVC sewer line outside the house. He thought if it was sewer gas, that by taking the cap off it would NOT allow the gas to get back into the house (He also covered the hole with a bucket, so I’m not sure about that theory). In any case, nothing has worked. Also, I’m not sure if this has anything to do with it, but the toilet clogs extremely easy. I use the plunger about once per week (continually warning my child to be very careful not to clog it up). If you could shed some light on this problem, you may save my marriage. Thanks!

It could be a dead animal in the wall.

Have you gone up in the attic to check for any open vent pipes that weren't tied back to a roof vent when the plumber did a smoke test?

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When the smell happens, it is always at night when there is activity in the bathroom

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At night the air is cooler and has higher humidity. This makes the lighter the air gas drift down so you smell it more. Smells and noises are the hardest for us to track down so about all we can do is give you a guess. Good luck, Tom