I have 3x CO detectors. We also have hard-wired smoke detectors.

1 in the master bedroom hallway (all bedrooms connect to this hallway)
1 in the basement (we have a wood stove, need to know ASAP if there's a problem there)
1 in the kitchen/dining area (we have a fireplace, that area needs coverage).

A few years ago we had ALL THREE of them go off at once when we got home on a cold night after a few days away, and lit fires in the wood stove & fireplace at the same time. The oil burner kicked on just as the fires were being lit. The slight negative pressure in the house caused by the oil-burner coupled with very cold-pipe chimney caused a momentary reversal of the draft and pulled CO into the house. BEEP BEEP BEEP BEEP - scared the crap out of me, as at the time I had no idea what the hell was going on, there was no odor at all.