They don't even need to get into anybodies router, WiFi signals are obviously wireless and can be analyzed by anyone in the area who has an antenna. Triangulation can very precisely pinpoint where a WiFi signal is coming from. There are devices that can instantaneously pinpoint the location of all devices connecting to a certain access point. http://www.flukenetworks.com/enterprise-network/network-testing/AirCheck-Wi-Fi-Tester These days even local police agencies have such devices. People are constantly being busted using their neighbors WiFi in the mistaken belief that it is impossible to trace back to them. Assuming you regularly spoof your MAC address, It is very unlikely that you will be identified as the person who used a given wireless access point if you are no longer engaged in a pattern of using it by the time the police arrive. However, it is still possible to narrow in on people in such circumstances. For example, the FBI identified Petraeus's mistress despite the fact that she always used WiFi at hotels, due to the fact that her name was the only one registered at all the hotels during the time when communications were sent. This is a sort of intersection attack that could make it possible to deanonymize someone over time even if you don't get them with a live trace. However, live traces are trivial to do if you are in the area.