Answer:
If you've noticed in System Manager that when you browse mailboxes the "Last logged on by" column frequently displays what seems to be incorrect information, you're not alone.

For instance, your account may show other people listed in the "Last logged on by" column. This is often very confusing for the administrators, as you've already discovered. Obviously, as an administrator you'll be concerned about security -- and finding several people being logged into other people's mailboxes is not very comforting. Rebooting the server won't make a difference and even if your server is up to date with all the patches and hotfixes, the behavior still remains.

The reason for this "feature" displaying other users as "Last logged on by" in Exchange is that the other users are querying free/busy time. They are technically not "logged on" to the mailbox in the traditional sense; they are merely accessing the calendar information, and Exchange decides to report that the mailbox was logged on by them. You can test this easily with a meeting entry by querying someone else's mailbox for free/busy time. You will notice that your name will show up in the "Last logged on by" column.