If you have removed the Quell CO alarm and removed the batteries (if the model is equipped with batteries), the CO may still have a slight charge.
To drain this charge, press and hold the test button for 10 seconds. If a beeping sound persists, there must be another device that is causing the beeping yet is less apparent than the CO alarm. Some such devices are a smoke alarm, a CO or smoke alarm in another location, or a cordless phone.