During the second Devil May Cry DMC Mission: Home Truths, you can collect two secret keys: Copper Key and Argent key. The Copper key is located inside the house in the first huge room.
You will need Elyx, which you can obtain in Mission 7, before you can obtain the key.
Secret Key 2 – Argent KeyLocation: Mission 2 – Home Truths
Required ability/weapon: Elyx (weapon, unlocked at mission 7)
Additional Information: After acquiring Arbiter in the event that happens in room with Sparda’s portrait, proceed towards the passage that opens. The passage will spread in length, creating gap between platform you are on and platform with a Lost Soul on it. Jump into the gap and proceed to the nearest room. The Argent Key is under the red crystal on ground. You need Elyx in order to destroy the crystal and obtain the key.