In the game's Top Secret menu is a section called The Hidden Door. There is an option that can only be unlocked by using a 3DS system-unique password found in the Hidden Door section of other Professor Layton games. It gives you access to a bonus conversation between the cast, as well as a brief quiz about the events that took place in the previous game:
Unlock the following options in the Top Secret section of the Bonuses Menu by performing the corresponding tasks:
OPTION - HOW TO UNLOCK Additional Character Profiles - Solve all 170 puzzles. Art - Acquire 4600 Picarats and beat the game. Character Profiles - Acquire 4300 Picarats and beat the game. Character Voices - Acquire 5200 Picarats and beat the game. Movies - Acquire 5500 Picarats Music - Acquire 4900 Picarats and beat the game.
Unlock the following bonuses by performing the corresponding tasks:
BONUS - HOW TO UNLOCK Layton Life (Japanese version only) - Complete story mode. Mouse Alley Mini-Game - Collect ten mouse badges by tapping mice that randomly run across the screen, solve the Luke Foretells mystery, then talk to Fische at the Crossroads. The Biologist's House - Complete the Laytonmobile Cove fish tank mini-game. The Boatman's House - Complete all ten Toy Train mini-games. The Flute Player's House - Beat the game. The Puppeteer's House - Accurately complete all three puppet shows. The Puzzle Master's House - Solve 167 puzzles.