Secret level?
Ok, this is just a guess, but my guess is that there actually is a secret level. Maybe if you complete the game at Silent Assassin difficulty and all the levels at SA awards you're able to unlock this secret level. Has anyone ever done this?
The motivation for my guess is plain and simple; In the mission select menu there is on extra blank bar underneath the last level. Also you've never actually eliminated the first targets in the last level.
If someone can help me out, i'd be grateful, thanks!