Our house is developing a cigar-smoke smell, and there have never been any cigars inside the house (not at all, much less lit ones) in the 5-1/2 years we've been here.
It's mostly near the cold-air return for the furnace (the ceiling in a hallway area outside the bathroom), and mostly when the furnace blower is off. We can't smell it coming out of any of the vents when the blower is running. The CO detector reads zero at all times until air from the garage gets in after we park the car, at which time it will read 30 to 70 depending on exposure.
Cigarettes were allowed in the house, but there haven't been any in over a year, and anyway this doesn't smell like cigarettes in any way at all. This is not the smell of smoke that remains on clothing and walls after smoking, this smells very much like dilute secondhand cigar smoke.