Any chance you've just installed Murano glass lighting? My stairs smelled like fish for the longest time. Finally traced it to the five pendant lights.
You could tell if the plumbing vents are blocked because your sinks won't drain very well. I don't think a crack in a vent would amount to much but maybe.
Dead mice in the basement ceiling/main level floor? Probably something dead. Maybe something outside and underground that is thawing out and heating up in the warm weather?
Possibly a toilet leak and actual sewage? That could accumulate slowly in the ceiling/floor.
Pictures?
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