Observation Log option in DCUI?

To prevent a potential security breach, verify that the login attempt was made by an authorized user.  You can check /var/log/syslog.log and /var/log/hostd.log for both successful and failed login attempts.  The /var/log/vobd.log file also contains logs for failed login attempts. This /var/log/vobd.log  is knows as observation log in ESXi.

Also Viewable at:-

https://YOURESXhostIP/host/vobd.log