First, what does the smell resemble? Sewer gas does not always smell like sewer gas. Check the traps under your basin and/or tub. If they are leaking they are allowing the odor through the traps; or they simply evacuated themselves. I've had situations where a trap leaked under a basin and the resulting mold under the cabinet smelled like methane. What I would do is clean the entire bathroom and wait a day. Then on my hands and knees, peform a "sniff test" around all suspect areas.
P.S. It could be a mouse. Those little buggers can get into the darndest places. I've actually drilled holes in each stud bay before I found them.
[This message has been edited by Iceman (edited January 24, 2002).]