Hello. We have an older house with a septic system. We live on the side of a mountain and the system drains down the hill. Anyway, today when my husband used the upstairs toilet and flushed, I swear I could smell sewage type smell in the living room. The pipe runs from upstairs toilet down the outer wall of that room. That is the only place I could smell it. No wet spots on the wall or in the basement, also no smell in the basement. What could be going on? Thanks!

Your entire home vents through the roof with a main vent stack. This vent is an open to air point for your sewer and its possible that sewer gas is rolling down your roof and getting inside, where any windows open? Try flushing other toilets to see if the smell returns each time.

No windows open, and no smell again, even after flushing the toilet after use a second and third time. Maybe something was blocking the vent like a bird or a leaf?

Could be but let us know if it returns