.smbdelete files accumulating on server share
Spotted an answer on the Apple support forums: https://discussions.apple.com/message/30046649#message30046649


  Apple introduced this behavior in OS X 10.10 you can find it in the
  source code here: 
  
  http://www.opensource.apple.com/source/smb/smb-759.40.1/kernel/smbfs/smbfs_smb.c
  
  The comment above the code that does it is:

* We have an open file that they want to delete. Use the NFS silly rename
* trick, but try to do better than NFS. The picking of the name came from the
* NFS code. So we first open the file for deletion. Now come up with a new 
* name and rename the file. Make the file hidden if we can. Now lets mark
* it for deletion and close the file. If the rename fails then the whole call 
* should fail. If the mark for deletion call fails just set a flag on the 
* vnode and delete it when we close.



Other googling implies that these are held while 'someone' has an open file handler and are cleaned up automatically later.