Help with stains on interior wall/ceiling

I inspected a home today and found some very odd stains on the ceiling in many of the rooms and an outside wall in one of the rooms. The stains are black to dark gray in color and are found on the wall and ceiling where the studs and joists are located behind the drywall (see photos). The stains can be wiped off with a damp rag. I went up in the attic and did not find and stains or moisture on the wood members or the back of the drywall. The attic has great ventilation. The house is 13 years old.
Does anyone know why this stuff is attracted to the studs like this?

It is a condition known as “ghosting” usually caused by, well, a lot of different things. Rather than me telling you I attached websites for you to read up on the condition and its probable and possible causes.

Here you go.

Thank you Doug.