CLI command openfiles is quite useful to know which files are being used by the system.

openfiles /local

Maybe we get this error:

The system global flag ‘maintain objects list’ is currently disabled

That we can solve in this way:

openfiles /local on

Also we can see remote system open files by:

openfiles /query /s [server*name] /u [domain]\[username] /p [password]

Files open by notepad.exe are not marked as open by the OS.