If your bath has a floor drain, it may have evaporated the liquid seal out of the trap; it can also happen to a seldom used shower or tub drain. To remedy the problem just pour a qrart of water in the drain the first day of each month.
If that is not the case, see if you can open a sewer clean-out outside, drop a small smoke bomb (from a plumbing supply store, its just a small thing like a cherry bomb, except it doesn't blow up) in and blow the smoke upstream with a vacum cleaner with the hose on the discharge side. Someone needs to be in the house to see where the smoke leaks out at the same time. Some use peppermint liquid in this test but it is very expensive and is still invisible.