Where is the temp file created by Disk Utility during erasure of free space kept on a non-boot volume? I used Disk Utility to perform a secure erase of free space on an external non-boot drive, and ran into a problem and the process was aborted.

I see that the file is still there because the drive capacity has taken a big hit. So I need to know where the temporary file Disk Utility writes to is.
Whilst I don't know where its stored, it is at least able to be removed by running Repair Disk in Disk Utility freeing the space for a subsequent free space erase.