Do you burn lots of candles? Any appliances that move air (refrigerator, microwave oven, etc.) will likely be effected by the soot produced by them. Staining by wall studs could possibly show where air is leaking through.
I think you can buy sootless candles which may help in the future is used instead?
BTW. Some candle wicks contain a strand of lead to keep them upright, which can the hazardous if used.