If dopus.exe is still in the Details list more than a few seconds after using File > Exit Directory Opus (not just File > Close Lister, or closing the window, which won't usually exit the program), something may be preventing it from exiting. That in turn might be related to what's locking the drive.
If that's happening, next time you see it, please make some process snapshots and send them to us. We can use those to see what it's waiting on, and if it's something in Opus itself or something in another component.