Folder in use - How to find what's using it?

Long time user of Directory Opus. First time with a problem.

I'm attempting to move a folder from one directory to another, and I get the error message: "An error occurred moving 'Reover Keys': The process cannot access the file because it is being used by another process. (32)

I cannot locate any use of the folder anywhere in my Windows Task Manager. How can I find out what process is using the folder? Thanks.

Have a look here for suggestions: