I tried searching on this thread first to see if this was mentioned, so I apologize if this was a repeat, but one of the blogs I follow mentioned there's an issue with Windows 10 being set up as a default to share your Wi-Fi login with anyone in your contact lists/Facebook.
http://krebsonsecurity.com/2015/07/w...with-contacts/
He does stress that it will ask (I think the first time, then not again), but includes what seems to me a convoluted way to turn this off from happening by default.