What's this newly-appearing-never-seen-before thing in Facebook Chat:
What's the new Seen  in Facebook Chat?
facebook chat
Seen means that your message was loaded to the browser AND was visible to the user.  If the tab was minimized or inactive at the time that the message was sent the message will not be marked as seen.  Messages are marked as seen through AJAX in a javascript function. I traced the function call, and it is being made somewhere on line 221 of http://static.ak.fbcdn.net/rsrc.php/v2/yz/r/7wj8FiAwzjS.js.  Of course it is minified javascript so it is relatively hard to read.If you are looking to disable the feature, a chrome extension has been created called Facebook Unseen.