Old question, but i'll have a go anyway.
Wifi enabled devices usually send probe requests to try to find Access points they previously have been connected to, even when they are nowhere near them.
If you're using backtrack/kali linux, try this:
Create a wireless adapter alias running in monitor mode (assuming your adapter name is wlan0):
airmon-ng start wlan0
Start scanning for devices and access points:
airodump-ng mon0
The access points will be listed first with their Mac addresses under "BSSID", followed by the devices which will have their MAC addresses listed under "STATION" and a "not associated" flag under "BSSID" if they aren't connected to an access point.