WiFi Sense is the new 'feature' which automatically shares your WiFi passwords with all Outlook, Skype and Facebook contacts who also use Windows 10. Microsoft don't tell you that they're doing this, so if you agree to Express Settings when installing or updating to Windows 10, WiFi Sense is silently turned on for you. To change your settings, go to Network then Manage WiFi: Settings > Network & Internet > WiFi > Manage WiFi Settings. There you can choose which contacts you share WiFi with. If you want none of it, you can uncheck every box.