re: Construction/ Home question. Help with fireplace/mold smell situation

If yall run range hood vent or fart fans often then you may be getting some humid warm air getting drawn in to replace the air being exhausted by the vent fans. I don't quite understand your situ but I'd leave some vent fans on and go explore around the chimney to see if you feel or smell exterior air.

Fyi, its not uncommon for chimney's and the space around the flue to be a leaky air pita.