Two east places to check: the toilet and bathroom sink drains...
Dies the toilet seem to hold just a little bit of water over the valve that allows it to dump? Or does it usually seem to be dry?
If the latter is the case: there's your culprit!
The sink should have some sort of check valve in place to prevent odors from coming up from the tank. See if it is moving properly, and not stuck open.
Good luck!