From the looks of it, the file responsible for this is /var/log/authd.log
Running a test I entered system preferences and access an admin required location and it recorded in that said file as my username being granted.
Thank you for the response, I see where it shows the unlock but doesn't log who was granted admin rights.
I am going to try opening a ticket with Apple to see if it can be done. Thanks again
There's also some information logged in /var/log/system.log. You could match the date/time of the system log entry to the details in the authd.log to get the complete information.