One way of scanning for live systems is Wardriving, the process of driving around with a wireless enabled device, such as a laptop, smart phone, tablets or other device with the goal of mapping out access points, usually with the help of a GPS device. If don't carefully and with some planning, you can locate many access points along with their configurations and physical locations. There are a number of tools that can be used to perform wardriving.
List of commonly used tools in this category...
AirSnort
This is a wireless cracking tool.
AirSnare
An intrusion detection system that helps you monitor your wireless networks. It can notify the system owner as soon as an unapproved device connects to the wireless network.
Kismet
This is a wireless network detector, sniffer and intrusion detection system that has the capability to assign GPS coordinates to each wireless access point discovered.
NetStumbler
This is a wireless network detector that is also available for smart phones and other hand held devices.
inSSIDer
This is a wireless network detector and mapper of access points. This tool is also capable of evaluating noise levels in the mapped area.
Warwalking is equal to wardriving except, as the name implies, warwalking is performed on foot while wardriving is done while driving.