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Imagine this situation that there are some smartphones and computer around with their WiFi adapter (wireless adapters) on, but not necessary connected to a network.
Is there a way to look the MAC addresses via a Linux machine?
You can use a tool such as kismet, http://www.kismetwireless.net/, to scan for wireless networks. I believe it can show you all wireless devices in your vicinity without having to have the devices being actively connected to same network.