Years ago I purchased a proper CO detector. I did that because my research showed that by the time those ubiquitous ‘dot’ cards stuck to the dashboard turn black, you’re likely already blacked/blacking out yourself. Further, many leaks start small - pinholes in exhaust systems that develop into cracks and other engine bay/firewall defects - and these create low CO levels in the cockpit that can nonetheless be detected by a proper CO detector before deleterious effects arise.
I was alarmed - literally - at the amount of CO that sometimes entered the aircraft during taxi, depending on wind direction and speed. And that was with windows open (in fact - that may have been the / one of the path/s of entry) and without smelling any strong exhaust smells. I’m talking levels at which protracted exposure would likely result in deleterious effects.
My eventual practise was to leave the detector switched off during taxi and take off, to avoid the distractions, and turn it on during the cruise. (The ‘off the shelf’ item did no have an on/off switch. I added that myself.) Fortunately it always read/s a big fat 0 in the cruise. I figured that the risk of a defect developing to ‘bad’ between a landing and the next taxi, take off and climb to cruise were lower than the risk arising from distractions from the alarm.
The combination of protracted exposure on the ground combined with ongoing exposure in the air due to aircraft defects? Not good. (My inexpert guess is that a pilot’s pulse rate and metabolism are running a bit faster in the initial phases of a flight than sightseeing passengers after a nice lunch...)
Modern and effective CO detectors are cheap insurance, folks.