It shows if the moderator edits a users posts. It has always been there, but most won't notice it until a moderator edits another users post. It looks like it's a language string so you should be able to add the below to your CB language files to get rid of it. If it already exists then set its translation to blank.

'<br /><strong>[Notice: </strong><em>Last Edit by Site Moderator</em><strong>]</strong>' => ''