first question on StackExchange ever. I hope I got the right board.

I'm trying to create a device (either from a standard AP or some other unconventional means) that will be able to detect nearby Wi-Fi enabled devices. For example, if a cellular phone (iPhone for instance) would be carried into the secured area, its MAC address will be logged. A cellular phone is a good example because it's the most common threat that should be detected.

Some important points:

1. The detection can be either active or passive, doesn't matter.
2. The detected device might be connected to a different network, or might not be connected to anything at all. I assume most cellular phones are actively probing when not connected, but I'm not sure.
3. It is important to not only identify the breach, but also to identify the device (MAC address).
4. Conventional hardware is only optional.
5. Distance of detection is at least 6 meters (20 feet). Handling one device at a time is good.
6. Speed of detection is important, under 5 seconds is ideal.

So my question is, is this even possible? If so, what can I use in order to make this a reality?

Thank you for reading!