It would be easy enough to only use the CO detector when the car is in park or the emergency brake is on. That way it could never activate while driving.
For modern cars, I think that's what's happening actually - engines will power off when stationary and not in gear, but the car is still on and the engine will restart when it's running low on battery power. Thus, people leaving their car in the garage thinking it's off.
If the engine’s off because of stop/start the warning noise should already be much louder and accompanied by dashboard warnings. See also what happens when you leave the lights on.
That’s what doesn’t make a lot of sense here before we dash off to design some sort of countermeasure - there’s either an engine running (and “modern” engines aren’t that quiet, especially in a confined space), or there should be some loud visual/auditory warning. I guess it’s possible the victims didn’t hear either?
I suspect keyless entry systems have prevented multiple deaths of children in parking lots, but there’s no way to track that.