brontosaurus is correct. In addition to delegates and basic calendar access, you will see Event 1016 in Application log: "NT User Domain/1stUser'sLogonName logged on to 2ndUserLogonName mailbox, and is not the primary Windows NT account on the mailbox":
1. If you have Outlook configured to schedule resources (such as conference rooms, laptops, etc.) this allows users to access mailbox calendars to (a) check another person's or a resource's availability, or (b) enter and send a booking request to mailbox calendars.
2. When (a) backing up a series of individual mailboxes using a program like ARCserve or (b) during an on-demand virus scan of the information store multiple mailboxes will be accessed, and you will see a system account as the repeated logon name in cases 2 a & b for the sequence of mailboxes in the log.