Low Power Proximity Sensor, Battery Powered

Using Arduino Sensors
3

  1. Some human-sized object? 802.11 a/b/g/n wireless? Bluetooth? My main concern here is battery life; I could go either way for the initial detection.

  2. The bluetooth boards I've seen require more power than the Arduino. If I could find a way to detect bluetooth signals and wake the Arduino / full bluetooth stack on that event, it would be perfect.