Why does the CO detector continue to emit short beeps every 40 seconds, even after airing.

This beep is an end of life signal.  CO sensors of most detectors have a lifetime of between 5 and 7 years.  They become saturated over time and must be replaced.

The CO detector gives short beeps

If you hear short beeps from a CO detector that is less than 5 years old, this indicates that the detector is defective. Replace the CO Alarm as soon as possible.