The Close Outside Files command in the Project menu is only available if the active project is managed and it has one or more outside files. An "outside file" is a file that is open under the project's tab, but is not actually part of the project. When a project is closed and reopened, outside files are not remembered. Use the Close Outside Files to remove all such files from the project immediately.
Files that are removed from a project are added to the File|Open and Project|Add to Project submenus that list recently closed files, unless the files were opened in bulk withProject|Open Folder or a similar command.
Project|Open Closed Files
Project|Remove From Project
Project|Remove Closed Files
Project|Close All Files