Google offers (or at least used to) an API which takes 4 or more WiFi Mac addresses (part of what is broadcast) and will return an approximate location. I think

Google offers (or at least used to) an API which takes 4 or more WiFi Mac addresses (part of what is broadcast) and will return an approximate location. I think the data is collected by the street view cars. If you move house and take your WiFi modem with you it causes confusion, hence the need for at least 4 different devices. The sampling rate seems slower than it used to be.