you can use the "DEVICE OFF LINE" alert from the EVENT POLICY
Ability to track devices that are un-adopted for a specified duration of time (user configurable) and raise an event or snmp trap. This feature allows monitoring systems to subscribe to this event instead of device un-adoption. This event can be viewed under “show event-history” (or) can be forwarded to syslog or snmp manager as a trap like any other event. (no additional configuration required other than regular configuration to enable the syslog or snmp traps).