The process of thoroughly examining the radio waves to find wireless network is called wireless scanning.
Wireless networks scanning has become quite popular, even among nontechnical people, also due to the so-called wardriving phenomenon. Wardriving is the activity of pinpointing wireless networks outdoors, usually driving a car and equipped with a laptop, a high-gain antenna and a GPS receiver.
There are two main types of scanning: active and passive.
Active scanning involves sending broadcast probe request packets and waiting for probe response packets from access points, taking note of the discovered ones. This is the standard method used by clients to identify wireless networks that are available nearby. The disadvantage of this method is that an access point can be configured to ignore the broadcast probe request packets and to exclude its SSID from the beacons it sends (hidden AP), so in this case, active scanning could not identify the network.
Passive scanning...