Timestamps:
0:00 - Key #1
2:10 - Key #2
3:45 - Key #3
5:41 - Assassin's Bureau (Collect Mythical Sword Joyeuse)
The Assassin bureau in Francia is located in Paris, but you need to find three keys to enter it. Finding the first key will unlock the side quest "Hidden" and mark the other key locations on your map. After entering the Bureau and earning your achievement or trophy, work your way through the Bureau to find a mythical sword, Joyeuse, and complete the side quest.