It could be, too, the smell of long dead decomp (like, mouse in the ceiling). We had one die in a building at college, and the smell was almost sweet, musty, animal, and a little pungent, all at once, and lingered for a couple of months (as they had no idea where it was to remove it). Have you had any rodent/bird incursions? If they are near the vent fan, the smell could be coming back in.