[hunt]

Anecdotally, I had a very strange experience with LinkedIn. I went to stay with my friend in another country for a few days and upon returning home, LinkedIn recommended his father as a connection - despite me never electronically contacting him. The friend I stayed with doesn't have LinkedIn nor have I contacted him over email or similar. My guess is that it must have been location based or because I connected to their WiFi - very weird.

[phy6]

Yes, my guess is either public IP or GPS colocation from phone apps, possibly even using the wifi AP name if you both had phones connected. If civilian GPS is good to a few meters, it's easy to look up who lives in that radius.