You can find air leaks in your house like that. Ventilating fans in bathrooms depressurize the room slightly and you will see a slight stain at leak points. In the attic, the converse is true. The slightly pressurized house will cause fiberglass insulation batts to get slightly stained from dust and odors getting blown out of the house through leaks.
Gross, about what I think of smoking.
“In the good old days of smoking on passenger airplanes, maintenance used to find weak spots in the fuselage by nicotine stains on the outside of the aircraft.”
There was some consternation when cigarettes were banned because the tar would seal the rivets and make the plane easier to pressurize. I think they ended up having to use some sort of sealer at the factory.