(a) People walking around near your house with Android phones are constantly Wi-Fi scanning their environment, correlate Wi-Fi SSIDs and MAC addresses with GPS

(a) People walking around near your house with Android phones are constantly Wi-Fi scanning their environment, correlate Wi-Fi SSIDs and MAC addresses with GPS locations, and sending this info to Google (b) When you or anyone else don't have a GPS signal because you're indoors, Google can run a Wi-Fi scan and geolocate you indoors from (a) Also, s/Google/Apple/, s/Google/Mozilla/, they all do the same thing. You can even run a Wi-Fi scan yourself with "sudo iwlist scanning", put the data in the appropriate format, and send it to Google's API [1] or Mozilla's API [2] for geolocation. [1] https://developers.google.com/maps/documentation/geolocation/overview?hl=zh-tw https://developers.google.com/maps/documentation/geolocation... [2] https://ichnaea.readthedocs.io/en/latest/api/geolocate.html#api-geolocate-latest https://ichnaea.readthedocs.io/en/latest/api/geolocate.html#...