In Exchange 2010 if you grant yourself full access to a mailbox then you will see that users mailbox appear in your Outlook. If you have not seen this behaviour before then you have probably upgraded your Exchange server to SP1 or above.
In SP1 Microsoft changed the behaviour for new mailboxes so that they automatically add to your outlook if you have full mailbox rights.
This is explained in the Microsoft Technet Article here
To disable auto mapping on existing mailboxes running the following commands in Exchange Powershell.
$mailbox = Get-Mailbox
$test = $mailbox | Foreach {Add-mailboxPermission $_.Name -User UserAlias -AccessRight FullAccess -Automapping:$false}