[agranig]

An option would be to connect via websocket to a server and react on the disconnect event (doing some simple ping-pong to keep the connection alive). That would require an always-on websocket server though, which is a different story. Still could be good enough for a gimmick like this.

[MasterScrat]

The always-on server could be provided as a (paying?) service, and there could be a polling fallback. That would be a lot more useful.