We have a half bath downstairs in a two-story home on septic. Occasionally (every 7-10 days) we get a strong odor in the half bath. It seems like sewer gas, propane, or a dead mouse. The other two bathrooms do not have this. This post suggest a wax ring issue which I will investigate.
I wonder if it could be something else though. I installed the toilet about a year ago and was pretty careful with the wax ring. This summer we had a partially clogged leach field. If we used a lot of water in a short time, say 3 showers in a row, and washing clothes, we would get some seepage out of the cleanout cover. I'm remediating it with an Areo-Stream and the field seems to have been flowing correctly for several months. When the septic system was clogged, we could hear the wastewater "chug-a-lug" through the pipes. Just wondering if the smell could be related to the septic issue.