During the sixteenth Devil May Cry DMC Mission: The Plan, you can collect one secret key: Gold key. DMC Gold Key can be found on the 105th floor, right behind the corner when you exit the elevator.
Secret Key 1 – Gold KeyLocation: Mission 16 – The Plan
Required ability/weapon: None
Additional Information: Between the platform where you are standing, and the one where the Gold Key is, there is a gap with flying cubes. Wait for the cubes to move away, before you decide to jump over.