Ok I'm not sure if the the OS knows if a network is secure or not. It would only display a message from your network. Windows does show all the connections within the range of you Wireless network and does show in the network is secure or not. My secure or not at that level means does it require a password to login. McDonalds, Panera Bread, Barnes and Noble are all examples of open networks that don't use a password. Hotel room usually have a password that the front desk give you at checks in. Now there are different levels of security that can be used but the OS only says secure or not.
Network security consists of the policies and practices adopted to prevent and monitor unauthorized access, misuse, modification, or denial of a computer network and network-accessible resources. These are outside the OS. The only way an OS would know whether a network is secure or not would be a message from the network or network tools. I work in IT and we use domain security if we login with the wrong password we are getting a message from network not the OS. We use a domain controller. We have a firewall and we also have servers outside our network which we call our DMZ which has its own security.
Now I found this on a MS page:
Projecting to a PC auto-discoverability is off by default. To have your PC beacon so it can be discovered and you can project to it through the Connect quick action from a Continuum enabled phone or another PC, go to Settings > System > Projecting to this PC and set “Windows PCs and phones can project to this PC when you say it’s OK” to “Available everywhere” or “Available everywhere on secure networks”. You can also set other settings on this page to your preference.
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Announcing Windows 10 Insider Preview Build 14385 for PC and Mobile