washing machine, toilet, dishwasher etc. were all not being used at the time. upstairs toilet bubbled up a bit with water hitting the lid but not overflowing or anything... at which point you could smell the sewage/methane gas. I have tried to reproduce it with no luck. visual inspection shows no clogged vents (can only see so far before it curves). had main line cleaned about 3 weeks beforehand. should i be concerned with this only occurring once or should i just let it go until it happens again? it smelled in the downstairs bathroom a little but no water marks on toilet lid though. thanks.