When I unmount a "Windows server" it disappears from Files

When you day "Windows Network", are you referring to the Networking category in the sidebar, or the "directory" view when you select the "Windows Network folder" (smb://) under "Entire network" (network://)?

It could be useful for the sidebar entry for a connection to persist even after unmounting so it can more easily be remounted.

There is a way of achieving the same effect in the branch lp:~jeremywootten/pantheon-files/fix-network-browsing by bookmarking a network root location. The bookmark persists after unmounting and activating it remounts the location (asking for a password if required).

I'm referring to the when you select the "Windows Network folder" (smb://) under "Entire network" (network://)?
Let's call that item "Windows Network" item as opposed to the "Windows Network" bookmark.

I think that there should be a place where all the network connections (mounted or not) are displayed. By network connections, I mean FTP, AFP, SAMBA... anything that the "Connect to server..." can create.
Displayed, removed and edited (and even renamed or copied and pasted)

It means that Files should be able to deal with special files/folders that are not properly files or folders.
I don't how hard would this be or if this already done in a way that is convenient (for the developer).

As to where those network connections should be located, that's a good question.
I have no idea.
Maybe another bookmark called "Existing connections".
Or maybe those network connections should be listed under the existing Entire Network item.
Un-Mounted network connections being disabled or having an overlay icon (and still being editable and removable)

In the branch lp:~jeremywootten/pantheon-files/fix-network-browsing, which may soon be merged, it is possible to create persistent bookmarks to network locations. Clicking these will cause the connection to be re-established although a password may need to be entered. For connections to samba shares there is an option for password to be remembered either for the duration of the session or forever.

It does not seem possible to have a view of all *possible* connections, but it would potentially be possible to maintain a list of the credentials for all connections that have ever been made and have a view of these.