Is It Mold?

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We've lived in our house for almost nine years, and have had a recurring problem in our central hallway. At one end, in which a few steps lead up to the master bedroom, dark (almost black) soot-looking stains have continuously formed on the ceiling and upper walls. On a couple of occasions I have partially (but not completely) removed these using a household cleaning agent and water. Then they form again! We've considered that this may be mold, except it doesn't look like any I'm familiar with. It looks like dirt or soot. We can't imagine where this may be coming from. Directly above is the entrance to our attic (a small removable panel) but no stains are forming on that, and it seems to be pretty well sealed. The only other explanation we can think of is we sleep with a floor fan turned on (mostly for white noise) and this might be blowing dust or dirt out into the hallway, which rises to the ceiling. Yet this doesn't seem like an adequate explanation either. Any ideas?