This idea would work really well if you implemented it in wireless access points since they are already listening for Probe Request packets from stations that aren't associated to their network.
This means that if anybody opens a stolen laptop in the vicinity of one of these APs it would be detected without requiring that the laptop be associated to the local network.
This means that if anybody opens a stolen laptop in the vicinity of one of these APs it would be detected without requiring that the laptop be associated to the local network.