You don't need a "perfect storm" for a gas furnace to leak Co into a house. In our new house 3 years ago the direct venting pvc pipes were not glued correctly and fell apart, introducing all exhaust into the house. Did not have detectors then but do now. The only thing that saved us was the bedroom window open all the time and the exhaust hood in the kitchen always on low or high. We smoke but only at the stove in the kitchen under the hood. Only ashtray in the house. So I guess smoking saved our lives lol. Cutting back and going to quit soon. That should be an annual check upon starting up your heating system, check your exhaust to make sure it is connected completely.
Jon
Jon