Curiosity
Open every chest in the game.

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Note: You DO NOT need the nomad and initiate chests for this trophy, just the standard and locked ones.

There are a total of 294 chests in the game. Chests are marked on the map, and you just need to go up to them and interact with it to open it up. Once you open it, you'll get some money. Some of them are locked though, see Gentleman Cambrioleur for more info on the locked chests. Here's a breakdown of how many chests are in each district.

La Bievre: 40 Chests
lle de la cite: 30 Chests
Le Louvre: 40 Chests
Le Marais: 44 Chests
Ventre de Paris: 40 Chests
Le Quartier Latin: 40 Chests
Les Invalides: 40 Chests
Versailes: 20 Chests
There's also a chest underneath Notre Dame that needs gears to openl; you find these gears by completing certain Paris Stories.