I recently stumbled on this site, offering cell phone detection solutions: http://www.libelium.com/products/meshlium/smartphone-detection/
Where they state that:
Users have to do nothing to be detected as the WiFi and Bluetooth radios integrated in their Smartphones (Mobile, Hand and Cell Phones) periodically send a "hellow!" message telling about their presence.
In addition, using this method they say, that they can record the mac adresses of nearby devices and a bunch of other information.
However I am suprised about this "hello" msg being periodically send by mobile phones.
What is its purpose and how does it work? Why would something like this be periodically sent to all WiFi networks? What is there to gain?
Is the detection device simply a WiFi router with added functionality to record these hello msgs or is there something more going on?