Can I carry out a hidden/stealth audit, without the users knowing?
Yes. You can audit Windows computers on your network from your desktop using the network scanning feature without the user being aware.
In addition, if you deploy audit agents, you may configure these to run in 'stealth' mode. Users will be unaware that the agent is being run unless they use Control-Alt-Delete to check the task list at the exact time the agent runs, which is only for a short time.