I hope you never do. It sounds like it is outside. Why the CO detector?
Couple of reasons. (1) Fumes can get in through the soffit vents in the overhang (2) Gas furnaces (one up, one down) (3) Gas line in the fireplace (4) Gas cooktop (5) Gas water heater.
When we bought this house, the home inspector and I went down in the crawlspace and discovered that the flue had somehow been disconnected from the furnace. The downstairs furnace was pouring combustion gases into the crawlspace (and of course from there into the ground floor). As the home inspector pointed out, this actually had two advantages: (1) it kept the crawlspace nice and dehumidified (2) it lowered the price of the house. Also the house itself was so leaky that there probably never was any danger. All the windows rattle and the siding hadn't been caulked since the place was built.
Did I mention that the former owner was an idiot? We did get the house at a real bargain - but we've spent all we saved fixing the place up.