User Activity in Database properties show access of users to your database. In mail, sometimes on would like to know, WHICH document somebody else read.
Even if you do not grant access to everybody, Full access Admins can read your mail.
I could imagine a $AccessedBy field similar to the $Updatedby, holding the last n names as a database property or Mail/Calendar preference.
It's paranoid, I know. I could not find it being illegal, though.