She is first of all an immaculate housekeeper which makes this even more puzzling! Their house has a "musty" smell, not pet, not food, not dust, not furnace, not plants, not smoke, vaguely sort of damp. They have checked drains, pipes, furnace, appliances, shampooed rugs, laundered everything, cleaned attached garage, searched closets, checked roof, cleaned all footwear, checked luggage, and everything else they can think of to possibly be the cause. They have put our activated charcoal and the smell remains. They do not want to mask it with scents, but rid it. Any suggestions ?