Which field are you using?
Last Boot under Node
Last Boot under System Information
Last Boot under Node Current Statistics
Last Boot has changed under Node
Also, what OS is the server?
Are you using SNMP for polling the device ? Check if SNMP service was restarted on the device. Hope it helps.
Last Reboot Time is dependent on SNMP Service on the device (it would have picked this time , hence a false alert)
Its a 2008 R2 server and it is being polled by SNMP. It is the Last Boot under Node. Should I be using a different one?