For me, the best way to identify all of the possible machines out there is to scan by IP Ranges, all possible ranges for your network. You can add the multiple ranges in a new Machine Group and supply admin credentials valid for your machines. The scan will show you which machines are scanned, and which machines are on your network that can't be scanned and why it can't be scanned. It could identify which machines are not being scanned or discover rogue machines, this will at least give you a list of machines to work off of.
that sounds just what I'm looking for, thanks very much for replying.