Exchange Administrative Abuse?

I have a feeling that one of our techs is abusing his admin rights by reading co-workers emails.  I know this can be prevented by creating another account with restricted access, but I was wondering if there's a way to prove that this person is reading co-workers emails?  I cannot simply restrict this users access without some kind of proof, so I'm wondering if there is a log that will show this sort of activity?
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"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

Go into the Exchange System Manager and drill down to appropriate mailboxes and view "Last Logged on By".  Look for something that looks off.  Also note that if anyone views someones calendar, it will show up there as well.

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