During the fourth Devil May Cry DMC Mission: Under Watch, you can collect two secret keys: Argent Key and Gold key. While the Gold Key is in the hidden room near the first Divinity Statue, the Argent Key is protected by a red crystal which can be destroyed with Eryx weapon, unlocked at mission 7.
Secret Key 1 – Argent KeyLocation: Mission 4 – Under Watch
Required ability/weapon: Elyx (weapon, unlocked at mission 7)
Additional Information: Follow Kat into a little passage with stairs. At the end turn right. Argent Key is behind a red crystal on the wall with a blue painting of Jimmy.
Secret Key 2 – Gold KeyLocation: Mission 4 – Under Watch
Required ability/weapon: None
Additional Information: While proceeding through the mission, you will reach a plateau with fountain and Divinity Statue. While standing on a fountain, look behind and you will notice a blue ring, where you can Angel Lift yourself. On the top of that building, you can easily notice hidden room, with a gold key inside.