Recently we encountered a case where an AutoCAD user was just opening a network shared drawing folder in Windows Explorer and that temporarily caused the files to be listed as 'opened' by that user. After about 15 seconds the files were 'released' and no longer listed as opened by the user. Before they were released no one else could open a drawing from that folder.
Just by clicking on a folder the .dwg files in that folder (not .rvt or .dgn - just .dwg) become locked.
They tried creating a whole new folder and putting .dwg files in it but received the same result.
The confirmed solution was to turn off the digital signature checker (Start/Run/AcSignOpt). That solved the temporary lock on the .dwg files on the network file share. What was causing the lock was that the digital signature checker was scanning each file to locate and validate any digitally signed drawings and flag the drawing with a Valid Signature icon if a digital signature is valid, or an Invalid Signature icon if the digital signature is not valid.
Since there were multiple workstations experiencing this issue a method was requested to turn the digital signature checker off in the registry that could be pushed to all workstations.
The registry key HKEY_CURRENT_USER\Software\Autodesk\Autodesk Digital Signatures was located with the "IconOverlayEnabled" Value name. The valid values are 0, which disables the feature, and 1, which enables the feature. The changes made to this value will take effect the next time the user logs off and back on.
Warning! Problems caused by improperly editing the Windows registry could render your computer/operating system unusable. Microsoft provides a wealth of critical information that you need to know about the registry in the Microsoft Knowledge Base. Use the Microsoft® Registry Editor only at your own risk and only after backing up the registry as well as the system.dat and user.dat files as outlined for your operating system in the Microsoft Knowledge Base. Additional information about the registry is also contained in the Help topics in the Microsoft Registry Editor.