I pass these marks everyday at work. They are on an exposed steel beam that is level with stairs I climb to my desk and graphics studio. They are shadows of items left on the beams and as dust collected around these and then the items were removed, these marks were left. Paint splashes, as CSI would inform you, give it a timeline and are obviously later than the dust marks. Today, after over 4 years of passing these marks on the stairs and thinking that I must record their presence before they are cleaned off, I finally got around to it.