What most people miss about CO detectors is that they don't go off until CO levels have been elevated for a while. At low concentrations it can take multiple hours to trigger the alarm; at higher concentrations it can take 30 minutes. Even at dangerous concentrations it can still take 5 minutes to sound the alarm. So you can hold your detector in front of an exhaust pipe for a minute and it won't go off, but stick it inside an upturned pot with a candle inside and it will definitely go off eventually.

The reason for this is to prevent common false alarms like CO emitted due to cooking or smoking.