Someone has a file open.

So, they moved the GUI in 2012r2 to show who has a file open.

I have no idea where it went, and I'm sure there are a billion ways to do it on the interwebs.

001	Get-SmbOpenFile | Where-Object -Property Path -EQ 'DriveLetter:\Folder\Path\Here'

Play with Get-SmbOpenFile and it's friends Close-SmbOpenFile, Close-SmbOpenSession, Block-SmbShareAccess.

Ton of utility there.