This is a pretty cool secret area that can be found in Paris. With Nico, when you first exit Vernon Bliers apartment and go down the stairwell, walk over to the skater. When he stops moving attempt to use the lipstick on him. Nico won't do anything but you have now unlocked the area. Go towards the park and take the exit next to it (Not the road at the very bottom). Instead of Nico saying "I considered leaving the area" a loading screen will come up and you will now be in the secret area. There is nothing to do here except try to open a few doors and chase the dog that runs around here. There's also a nice view of the Eiffel Tower as well as being able to cross halfway across a bridge over a river.
Beat the game once in order to unlock the ''Extras''.When you finish the game you'll be prompted to save your finished game,confirm this.After returning to the main menu the an additional option,the ''Extras'' option will appear below the other options.The ''Extras'' provide concept art and other stuff referring to the Broken Sword games.