MicrosoftRegarding computer accounts, if they're domain-joined and on the network, they'll show logon events within the month, at the very least. Computer accounts change their passwords every 30 days by default and naturally they logon to do so.
Also, in most environments, group policy applies to computers - they also auth to receive their GPO updates. There are lots of processes on a machine that means they logon to the domain at regular intervals, nothing to do with bootup times.
If you want to see if a server is in actual use by clients, you'd be best looking for logon events on the boxes themselves, assuming they're not using apps that don't have windows logons.
Or there's always the "scream test", where you turn it off and see how many calls you get. Highly NOT recommended except after much monitoring and audit of all apps on the server and each app's activity logs plus the Windows security logs.
