On some interior walls in my house, I'm noticing dark patches that
seem to be appearing between studs. The walls are sheetrock that have
been painted a light color, and in several places, darkish, smokelike
color is emerging through the paint. Most of them appear high up, near
where the wall meets the ceiling. In the largest spot (about 3'
high), I can clearly make out where two studs straddle the dark area,
as if whatever is causing the darkening is trapped between the two
studs. What could be causing this discoloration?

-Fleemo