I have been randomly getting these periods of time(a day or so) where I enter my basement and I get hit by a strong wave of what I think is sewage gas. I recently replaced the wax ring underneath the toilet in the bathroom(which did have a slight leak prior to putting a new wax ring). I checked to make sure that it wasn't leaking again and it seems fine. There is a little Crawl-space next to the bathroom downstairs that has the main sewage line access but it looks fine. The only other thing I can think of is the floor drain for the water heater and furnace condensation.