Soot-like ceiling marks

I inspected a home today, where the owners complained about black, soot-like marks on the living room and kitchen ceilings. They have a six-year-old home with high efficiency gas furnace. The exterior furnace intake pipe drew much less air than the exhaust pipe exhausted. Is that natural? The furnace is in a small bedroom-sized basement room that is kept closed to keep out family pets. I thought the furnace might be starved for air and giving off soot that has collected upstairs. I tested some of these marks with a moisture detector. They were dry. I also used a thermal camera. The marks were somewhat cold. Any ideas?


I agree ghosting , also did they burn a lot of candles certain kinds put out soot.

I think they are clean on the candle issue. They had their kitchen and living room porfessionally painted, including a cathedral ceiling, where most of the marks are. Thanks guys. I appreciate the help.

Obviously not “professionally sealed”…

Agree with Dave, usually a stain blocker is used before final coat.