Mission 4: Under Watch
Proceed down the stairs just after the Kat tells you the city is trying to stop you and make a right at the bottom. A wall will shoot towards you and you will need to use the Eryx to break the red stone on it. Follow the alley to the end to find the key there.
When at the open street area where the divinity statue and fountain area, hop on top of the fountain and then jump and air dash toward where you entered. There will be a grapple point on top of the building across from you that you can snag and pull yourself up on. Once up, run through the building and follow it around to the key.
Can be found just after the battle with the Frost Knights where you find the two cameras. Use the grapple to reach the ledge above and then hop over to the right and cut through the blue door to find the Copper door inside the building
Near the end of the mission, after you pull out the stairs and find the last camera, air dash to the left and you will see a grapple point where you can latch to. Pull yourself there and you will land at the Argent door.
Mission 5: Virility
After the battle with the hell knight once you go through the red container, hop over the fence on the left side to find the key
When in the floating container room, go to the far side and hop up the broken stairs and go to the opposite side. Follow the grapples around you will eventually slide through a container to a platform. Continue across the grapples, air dashing the larger gaps and you will come to land on the platform with the argent key at the far side of the room.
Just after the divinity statue, hop on the boxes to the left and air dash across to the right to find the Copper door on the ledge above.
While running through the corridors with the yellow pulsating tubes, you will eventually come to a red door on the left hand side. Smash through it and you will find the argent door there.
Mission 6: Secret Ingredient
As soon as you are in control and in the yellow tunnel, turn around and you will see the key directly behind you.
Once you reach the bottom of the yellow tunnel and enter the hatch, exit to the platform with pillars on it and the door will be located on a small platform to the left side.