Project not showing as checked-in to TFS in Visual Studio 2013

You probably need to bind the project to source control.

• Open the project in Visual Studio.
• File > Source Control > Advanced > Change Source Control (or you might find there is only an option like File > Source Control > Add to Source Control - I can't remember exactly what this looks like)
• If you get to the Change Source Control dialog, then select all the projects in the list and click Bind. If this works you should have a tick in the "Connected" column and "Valid" status, and you can OK the dialog and your project should be good to go. If binding fails, then Unbind everything and try binding again.

Looking back to basics, I discovered some of the files (such as the project file itself) had never been added to source control. I discovered this by going to Source Control Explorer, selecting one of the projects unmarked in Solution Explorer, right-clicking to get the context menu, then selecting Add Files To Source Control, then selecting the whole list presented, then clicking OK/NEXT/FINISH. That marked the unmarked project in Solution Explorer with a plus sign (+) to be added. Then again from Source Control Explorer I selected the solution and clicked Check In Pending Changes. Then I got the lock icon in Solution Explorer.