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.
You want to make sure that all your traps have water in them. If your floor drain is dry, it will leak sewer gas. Also, if you have natural gas or propane, check to make sure that there are no leaks. Natural gas can smell like sewer gas.