When an Excel file is locked on a network share, how does it know who locked the file?
Excel creates a hidden file on the share called ~$filename where filename is the original file. That hidden file contains the name of the user who has the file locked. The name comes from the Office options, as others have stated.
For some reason that file was not deleted properly when the file was closed. Oddly, everything still works - the next user to open the file can still edit and save, but it looks like it's checked out to someone else. And even after editing, saving and closing Excel, that hidden file is still not deleted.
After manually deleting the file, everything works properly again.
Change the name in Excel for the users to be the actual users name under file>options>general