One of the new features in Windows 10 is called “Wi-Fi Sense“, which is supposed to simplify access to Wi-Fi hot spots.
At first, this sounds like a good idea… when there is an open Wi-Fi hot spot visible, it will simply connect to it, no fuss, no muss.
However, there is another feature, where you can be automatically connected to password-protected Wi-Fi networks, if any of your “friends” have connected to it in the past. Convenient? Perhaps. Security risk? Definitely. After all, the sharing works both ways.
That is, if a computer with Wi-Fi Sense enabled connects to your password-protected network, your password will be shared with all of that computer’s “friends”.
Here’s an article on “Ask Leo” that explains things further.
So, make sure not to share networks with your friends, and if you ever put your network password in for someone else’s computer, follow the directions in the above articles to make sure that they don’t share your network with their “friends”.