Bluetooth Low Energy. Unlike old bluetooth, this new service shuts down all the time to save power. In fact, apple recommends geofencing any bluetooth beacons you know will stay put, so it only starts scanning for nearby devices over bluetooth once you're within a certain radius. In this case, the app only looks for devices when you launch the app, though if you were willing to put up with a delay, you could do so on-knock. Haven't looked at the app yet. One more alternative: it could send the message and have your mac constantly looking for messages when locked. Third alternative, it pairs in the background and only once iOS notices the new device does it start actively opening communication channels. Apparently connect times are a few ms? http://www.connectblue.com/technologies/bluetooth-low-energy...