Warzone’s Rebirth Island Reinforced replaced the Security Area POI with Stronghold, and you need to head to the office in Stronghold’s circular building to deposit the keycards.
There will be a desk in the center of the room with lockboxes covering the walls. Each wall will have a different lockbox that you can open with keycards found around Rebirth Island and will give you some top-tier rewards.
Warzone’s Forgotten keycard can be found in the showers in the basement of Rebirth Island’s Bioweapon Labs.
Head right down the stairs until you reach a room filled with chemical barrels and showers, and you need to follow the prompt to ‘Turn on the Shower.’ Once that’s done, cash will fall out of the showerhead, followed by the Forgotten keycard.
Bring this keycard back to the Stronghold POI, and use the card to open the rightmost lockbox. You’ll get a Specialist Token for each member of your squad, a Loadout Drop marker, and the Unholy Advantage Calling Card.
Warzone’s Vikhor keycard can be found inside Rebirth Island’s yellow door vault, sitting on the desk underneath the map.
Luckily, you no longer need to complete the yellow door easter egg, as the door is now open by default. Simply head downstairs and pick up the Vikhor keycard from the desk, and bring it to Stronghold.
Using the Vikhor keycard in the Stronghold POIs middle lockbox will give you a free Loadout Drop marker and cash.
The Perseus keycard is the first and easiest keycard to get, as it’s simply found on a shelf inside the small building next to the Nova 6 Factory.
Bringing the Perseus keycard to the leftmost lockbox in Stronghold will grant you an Advanced UAV, as well as a big pile of cash.