I have a longstanding habit of saving my .sublime-project files in the root of the actual project, which may or may not be correct. Because I code on multiple machines and sync them with GitHub, I like to have my path be relative to the project file, so usually my project folder in the sublime-project file will have "path" set to "/"
With ST3 Build 3021 on Windows x64, the "close project" field is grayed out if I have the 'path' field set to "/". Setting the field to an empty string ("") prevents the folder from showing up in the folder list, but the project does save.
For reference, my sublime-project file is the following: