Once airborne, jet exhaust is not going to get into the cabin air system. Even on an engine with a problem, if it is running and producing thrust, the bleed system air is tapped well upstream of the fires and exhaust products of combustion.
On the ground, ingestion of fumes from other aircraft, ground equipment, or when the wind and aircraft orientation is just right, from your own engines or APU exhaust can happen. In any case, the fumes will be highly noticeable by most people, whereas CO has no odor.
It is possible there was something foreign to the aircraft in the cabin or cargo hold that produced the harmful gas. Failing oil seals have been known to affect cabin air, but CO would not be present.