This is just a theory but you would possibly use a wifi presence sensor to monitor it's fixed IP and react to the "presence" of the Xbox?
I use [UPDATED] iPhone WiFi Presence Sensor for my phones. Another avenue for investigation is the [RELEASE] Web Pinger (it monitors URLs so unsure it can check Local IPs) but that can activate/deactivate a virtual switch when a disconnection occurs.
Again, these are just theoretical suggestions. Maybe somebody has a more concrete idea and can share how they do it. @aaiyar's suggestion is a fool proof way!