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Why not incorporate a mini PIR into a phone. It could be enabled, for example, if you use your phone as an alarm to wake up. If you accidently press 'off' instead of 'snooze', the PIR would activate after a couple of minutes. If the PIR detected no activity (because you are asleep), the alarm could be activated again..
Then, you would never sleep through your alarm again!
It could also be used for a proximity alarm, work through bluetooth etc etc