@Susan3965 , Im guessing you thought of this but I will mention it anyway, it isnt an egg rotting smell that is a precursor leak detector stink (Ethyl Mercaptan most often is reported to smell like rotten eggs or sometimes rotten cabbage) added to Propane and Natural gas? If you even suspect that could be the case, you need to find out ASAP.
Otherwise 2 memorable times before that Ive experienced a lingering odor in my or adjacent spaces, Dead rodent in the wall (happens occasionally in a 200+ year old house) and Kimchi my barracks mate Beatle Heath used to eat every here and there back in the 80's, I know folks love it but somehow it didn't ever sit well with my sniffer (same with some of the really gross cheeses my mom used to love, RIP Mom).
Good luck with the stink, Im interested to find out what it turns out to be. Stay well, JR