contrary to other dangerous things in the cabin air, carbon monoxide is included in the certification satndards and measured during the flight testing of the aircraft.
Number one source for CO is recirculated engine exhaust when running on the ground with adverse wind. There should basically be no CO in the engine upstream of the bleed air system. Same with APU, so unless some of the intake/exhaust ducts do leak, it is highly unlikely to have CO in the bleed air in flight.
On the ground it can be a totally different story
Something smoldering somewhere may produce CO, oven insulation, bleed system contamination, electric fire...