Frequent tipster Danny Korem sent in a tip that can help you with AutoCAD files that remain locked even though they're not open.
"Many times I hear one coworker sharing a project say to another, 'Please save the file and close it, so I can have access to it,' and the answer, 'I was done with it 10 minutes ago.' In that case, look for the DWL (drawing lock) file in the same folder that the file resides in. When a file is closed by the user, AutoCAD should delete the lock file by itself, but sometimes it needs a little help. Don't worry, the operating system won't let you delete a lock file if the DWG is in use. As far as I know, DWL files don't contain any contents, and AutoCAD opens them to avoid non-singular access to drawings."
Notes from Cadalyst Tip Reviewer Brian Benton: Ever since AutoCAD 2000 (I believe that's the correct release, but feel free to correct me in the comments if I am wrong), AutoCAD has automatically generated a DWL (lock file) when a DWG file is opened. This is to prevent multiple users from opening and editing the same file simultaneously. Sometimes, especially when AutoCAD crashes unexpectedly, the DWL and/or the DWL2 files will remain, which could prevent users from opening the files. If this happens, then delete them. If you try to delete them while a user has the file open, you will not be allowed to. The DWL file is a plain text file and the DWL2 is an XML ASCII file. There are programs that can read the information inside them if you're interested; essentially they log who has opened the file and when. If you want to know who has a file opened, type WhoHas on the Command line in AutoCAD and browse to the file.