Broken Sword: The Smoking Mirror - Remastered (PC)
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Beneath A Steel Sky Easter Egg
Go left when you are in the London Underground and get the chocolate bar from the vending machine. If you then turn right you will encounter a ghost. If you show the chocolate bar on the ghost immediately before it disappears and then go left again and enter the tunnel you will be able to watch the scene featuring Robert Foster from 'Beneath a Steel Sky'.
Comics
Rescue Nico in the warehouse to unlock digital comics (accessed from the main menu) which contains the prologue of the game.
Diaries
Beat the game to unlock Diaries (accessed from the main menu) which contains character's complete details about their adventures.