The crucial question is "does Windows 10 automatically the passphrase for every WiFi network with Microsoft any time someone with a Windows 10 connects to it?"

The crucial question is "does Windows 10 automatically the passphrase for every WiFi network with Microsoft any time someone with a Windows 10 connects to it?" This may be happening, regardless of users' sharing settings, and regardless of whether the WiFi network has `optout` in its name. I haven't seen this possibility discussed anywhere, and it would be a good idea to try to figure out exactly what happens when a Windows 10 computer encounters a WiFi network.