Quote Originally Posted by YZ View Post
The files are on a server, but there is only one other AutoCAD in our office and their desk is right next to mine. I know that they are not accessing my files.

I had not considered that it could be a Windows thing and not a CAD issue. However, I have never experienced it except in AutoCAD and even then only in 2012.
I have experienced the same issue in the past.
The simplest way to solve this problem is shutdown and restart your AutoCAD.