We have a bathroom in the basement that periodically gets a sewer-gas smell. And yes, I do keep the traps full. Occasionally I even pour bleach down them. We use the bathroom fairly regularly, so sufficient water flows through the pipes I don't believe the bathroom was original to the house, but it's probably been there a good 20+ years. The bathroom has a sink, toilet, and shower. We have new-ish (10 year old) a septic system. The smell seems to visit us for a few days/weeks then goes away for weeks to months. We passed most of the fall and into winter without noticing it, until the health department did their yearly inspection on our septic system. Since then (about 4-6 weeks) we've had to run the exhaust fan 24/7. I was going to replace the wax seal on the toilet, but the toilet is grouted to the tile floor. Not wanting to break out that grout, I have left it in place. Two plumbers have looked at the problem and scratched their heads. Any thoughts on what to check?