Arrivals, but not departures

FriendTrack notifies you when a friend arrives in an Area of Interest, and when they change floor. It never notifies you when they leave one. The Area Alerts row says exactly that on the settings screen — when friends arrive in an area or change floor — and there is no switch that turns departures on, because there is nothing to switch: leaving is not treated as an event worth interrupting you for.

The reason is that an arrival is information and a departure usually is not. She is at the bar is something you can act on; she is no longer at the bar tells you nothing about where she is, and it would arrive constantly — every walk across a festival site crosses in and out of several Areas, and the edges of an Area are fuzzy enough that a person sitting near one can drift across it repeatedly without moving anywhere.

Departures are not hidden from you, though. The map and the Friends list always show where somebody is now, so a friend who has left the bar simply stops being described as at the bar the next time their Position (what other apps call your location) arrives. If you want to know where somebody is rather than be told when they move, that is the map’s job — reading the map.