Nice job, T. Saves me from a lot of typing. I deal with this problem a lot. I don't think the article mentioned the fact that filtration soiling can be an indication of a problem with your furnace. Just thought I'd add that.
The residue I'm refering to is on the walls, I'd swear it was candle soot, of course the tenant says they don't burn candles. They never want to admitt it because they fear being charged for the clean up and re paint. Although I can't prove it. I just cleaned it up and repainted the areas.
It is possible that the fresh air intake for the furnace/heater is drawing air that contains candle smoke or even greasy kitchen exhaust and redepositing it thru out the home by way of the other vents.