I have seen this question on forums and experienced the joy myself: you want to move / copy / delete the physical files for a database, so you detach the DB using SQL Server, only to find that you can't manipulate the files in Windows afterward because you get a message that the files are locked or in use. Tools like Process Explorer don't show any open file handles, though, so the Windows message that the files are open is a red herring.
Here's one piece of that puzzle: when the files are detached, probably as a security feature, the permissions on the files seem to be set so that only the principal that did the detach operation can move / copy / delete the files. Other people, domain admins or otherwise, can't manipulate the files.
So here's a scenario: I connect using Management Studio with Windows Auth to my server, with one account. I detach the database. The files are now set with permissions exclusive to that specific Windows account. Next I remote into the server using RDP under another identity and go to move the files. Let's say I am a domain admin at this point, just for fun. No go. Nada. Ixnay on the Ovemay.
In order to manipulate the files, I actually have to right-click/properties on the files, open the security tab and grant myself permission to manipulate the files. That generally fixes the issue.
