Sources of CO
I don't see any other sources of CO on a plane apart from the engines and the apu.
And three points. Planes burn a huge amount of carbon per second. So the % CO may be small but the absolute amount is big (compared say with a water heater, known killers).
A jet engine working properly might produce little CO, but what if it had a problem?
And finally, these pax did not die. The level of CO must have been tiny.
So where else to look?