Access to another Exchange 5.5 mailbox.

From my Exchange 5.5 logs I can see that Kevin (not real names)is accessing John’s mailbox. Kevin has no admin rights and John has not delegated Kevin to his mailbox. I tried to use Kevins Oulook 2000 mailbox to access John’s mail with no success. The logs are still showing that Kevin is accessing John’s mail. How do I prove that Kevin is doing this? And how is he doing it? Or is exchange reporting the wrong things?
Thanks in advance.