I would suspect that you were flying pretty precise orbits, and you simply flew through your own exhaust from the previous pass. If you had cabin air or heat elected, it pulled in air with exhaust in it. Once you trigger a CO detector, they can take a while to clear, so you could have been straight and level for a while before it recognized it was in clear air.
While flying a Cessna 207 with very advanced atmospheric resaerch equipment operating on board, I flew through my own exhaust of probably 15 to 20 minutes earlier, and the sensor went wild. The polution stays in the air for a long time.
It that situation I would not worry about safety. On the other hand, CO in the cabin for a period of 15 minutes or more is a serious issue, requiring definate "clear the air" action without delay. Flying an aircraft with het muff cabin heat and no effective CO detector is very foolish.
Pilot DAR