I feel like I must been living under a rock to not know this, but watch out for those Yankee candles and other scented candles because they can leave soot residue on your walls and ceiling.
Right before Thanksgiving, while in my cleaning frenzy, I noticed how dark the walls in our half bath had become. It was right near the area where we keep a large candle (or so I thought). When I washed it, this black mess came off and left horrible streaks. Off to Google it: soot from candles.
I bought a soot sponge at the hardware store, which helped to some degree. However, it was over all the walls and ceiling, as became apparent when I took the paintings down. I tried every suggestion and even Mr. Clean Magic Erasers left streaks. I just finished washing w/ TSP this week and painting this a.m.(not something I enjoy at this busy time of year!)
No more candles here! Thought I'd pass this along to save others the work I just went through.