During the eighth Devil May Cry DMC Mission: Eyless, you can collect one secret key: DMC Argent Key. Argent Key can be found in a hidden room, right after the subway tunnel.
Secret Key 1 – Argent KeyLocation: Mission 8 – Eyeless
Required ability/weapon: None
Additional Information: Following subway tunnel, will finally bring you to a big rectangle room, where you should defeat a few creatures. After defeating the creatures, don’t turn left in order to proceed with the mission, but jump on a subway tunnel. There will be a hidden entrance to your right. Destroy the blue vine in front of the entrance, in order to acquire Argent Key at the end of the room.