Why are some sensors, such as smoke detectors, not eligible for Sensor Activity Monitoring?

A sensor is ineligible for Sensor Activity Monitoring if it always sends an alarm signal when tripped. Smoke detectors, fire detectors, and carbon monoxide detectors, for example, will put a panel into alarm regardless of the panel's current arming state. Since Alarm.com will always notify the customer during these events, Sensor Activity Monitoring is not necessary. 

For more information about Sensor Activity Monitoring, see Turn sensor activity monitoring on/off.