Actually CO is produced from burning Hydrocarbons at too low a temperature, I tune Industrial Gas Turbines for low emissions for a living, and as an ex aircraft propulsion Tech i know a thing or three about Aero Derivatives, it seems unlikely that high CO (in excess of 50ppm) is produced from combustion in an Aero but it all depends on temperature really. As for how it would get into the cabin, i cant think of any mechanism unless there was an oil fire across the labyrinth seals of a bearing that would cause CO to enter the cabin. Oil would burn inefficiently and cold enough to produce tremendous amounts of CO - certainly in excess of 1000ppm