Network and Port scanning with Angry IP

It is important for a network manager to know what systems are running on the network.Rhetorical questions...

• What is authorized?
• What systems are on your network?
• How would you know if there was an un-authorized system on the network?
• What applications are running on the systems on the network?
• How would you know if an application was installed on a system on the network?
• Is it possible for someone to install a system on a network and not be notices?
• Could someone install a virtual system on your network using the built in HyperV that comes with all Windows systems?
• Is it possible for someone to install an application for test purposes and forget to un-install it?

There is the adage, Trust but verify. The organization hires good people that you trust, but how would you know if an administrator made a mistake or if someone was using the organizations resources for nefarious purpose?