We have meters that can alarm, mostly for tracking CO2 and humidity as the house is a bit too tight (from blowdown door tests). Got up and looked. We're between 1 and 2 ppm CO - but thanks for the PSA.
Also (trivia) I think the CO headache isn't so much from no O2 (although that's what kills you) but rising CO2 tissue levels. Not a doctor but when hemoglobin picks up O2 it changes shape, making it drop any CO2 being carried so the O2 in your lungs makes hemoglobin drop CO2 and giving O2 to tissues makes hemoglobin better at grabbing CO2 to carry back to the lungs. If CO locks up hemoglobin it isn't carrying O2 inbound *or* CO2 outbound - and having experienced low oxygen (private pilot) I can say there isn't a headache from insufficient O2, just fade-to-black. To be fair, there is some CO2 removal by diffusion into the serum but CO has got to be impairing some of that transport - so higher CO2. Any medical people want to set me straignt on this one?