A dry trap is the most likely but they do tend to have a distinctive smell.

If you have someone with a sensitive nose sometimes they can discriminate between different causes.

If it is a dry trap, it doesn't happen suddenly and it will always recur regularly. Pouring water in helps, but you have to figure out the interval and remember to do it quarterly, monthly, weekly, whatever; depends on temperature and humidity usually. Some people put a tablespoon of vegetable oil on top of the water on the theory it prevents evaporation in the trap. Maybe it works, maybe not, I dunno. I just make sure I run water periodically.

I have also run into a case where the exhaust fan was strong enough to pull some sewer gas up through the trap even though it wasn't dry. That was solved by putting a vent in the door so the makeup air could come in that way..