Heres several links that may be of some interest to you regarding your LAN audit.
Granted each has it's pro's and con's. Being that no OS has been specified I will post as if you are on a WinOS.
Shadow ScanN-StealthNeWTRetina Security ScannerIt may also be of interest that you monitor whats happening so you can see the scans happening and record it for review.
Theres quite a few others out there such as NMAP or TT, use what fits the project.
Theres some good reading material out as well. One I would point out for starters would be
Know Your EnemyHope that helps you out.