When Google’s “streetview cars” roamed around the world, they were performing wifi scans of their surrounding areas. By mapping the MAC address of your location to the van’s GPS coordinates, they built a map of locations that can accurately find your location, just by knowing the wifi router you’re connected to.
Google claimed this was “by accident”, and many countries objected to the collection of the data and the invasion of privacy.
As of 2010, Google had collected 600 gigabytes of data from over 30 nations. That list is now growing, as each new Android phone that gets purchased has the built-in ability to transfer locations of wireless networks to and from Google’s servers, to help aid in location services on the phone. The unfortunate part is even if you aren’t using these services, Google is still collecting this data via your phone.