Steal the key is a mission objective in Assassin's Creed: Unity (ACU)
"Steal the key" is a mission objective in Paris Stories / Flamel's Secret: Denis Molinier
Walkthrough[edit]
Make your way back down to the second floor. Head outside using the window on the Southern side of the house. There are 2 Courtyards inside the restricted area. You are going to want to find Denis Molinier in them. He moves around with a light guard of Extremists.
When it comes to Denis Molinier, you can either pickpocket him using any of a number of opportunities from blending or hiding places. Just take a little time to watch him move and it shouldn't be too hard.
If things don't go well, you can just take out the guards and Denis Molinier. Be sure to loot Molinier. He has the Key that you need to open the chest on the third floor.
