If we could detect MAC addresses ove the local wifi connection, with an occasional ping, like arp-scan, it could be used for alarm system setting, where all known mobile phone MAC addresses would trigger the shutting down of a security system.
The triggers could also be used to notify when a person is within range, or even as a time clock system for employees.
Show all MAC addresses on the network, detect all those on a friend list. If a friend disconnects, ping them to make sure it is not a device going to sleep. After two missed pings, flag that friend as off the premises, triggering whatever notification you want.