If the lights on the Airbeam2 are off, it could mean several things.
If the device's power switch is off, all lights are off unless the unit is charging. If the device's switch is off and the unit is plugged into the charger, no light means that the AirBeam2 is fully charged.
If the power switch is on, the lights on the base of the AirBeam2 turn off after a while to save battery, but the unit remains running. If you listen carefully, you should hear a fan running. This means that the AirBeam2 is on. If you cannot see a light and cannot hear the fan or any noise, the sensor has likely run out of battery and will need to be charged.