this will sound crazy, but we had tenants who swore there was a sewer gas smell coming out of their bath drain. (i smelled it too--it really did seem to be coming out of the drain.) several plumbers looked at it, trying different things, but the tenants still complained.
my handyman finally diagnosed it as the mildewing, rotting sub-floor under the ceramic tile. he ripped out the floor, replaced it, and voila: no more smell.