Watch the video below or follow the instructions to find all the car keys in the game:
Chapter 3: Sudden Impact - Find the car key on the CELL corpse in the stairwell leading out of the garages (video time: 0:00)
Chapter 4: Road Rage - Before getting the ICV, head past the turret and locate an open cargo container. Inside is a key on the floor (video time: 0:31)
Chapter 5: Lab Rat - Head to the end of the freeway and you should find the key by a dead soldier and a wrecked car (video time: 1:02)
Chapter 6: Gate Keepers - Once the window washing platform falls down, head left into the alley and find the key on the metal stairs in the corner (video time: 1:37)
Chapter 7: Dead Man Walking - From where you start the level, head down the road opposite you. Find the key by a tree in the corner down at street level. There is a teddy bear next to it (video time: 2:19)
Chapter 8: Seat Of Power - At the train station, look for the dead man sitting in the seat over the tracks. He has the key next to him (video time: 2:44)
Chapter 11: Corporate Collapse - Right at the start of the level, go into the downed train carriage to find the key (video time: 3:15)
Chapter 12: Train To Catch - As you pass through the vehicle decontamination garage while assisting with the evacuation, locate the control room to the side. This is where the key is kept (video time: 3:38)
Chapter 15: Power Out - Once you are on street level and heading for Times Square, search the trailer to the right to find the key (video time: 3:59) Chapter 18: Out Of The Ashes - Before boarding the vehicle to go to Central Park, go along the the flooded road a short way, keeping an eye out on the right for the key (video time: 4:20)
Chapter 19: A Walk In The Park - Following the path at the far edge of the park, find the key in the coach to the left past a run-down cafe (video time: 4:39)
Find this Easter Egg during the '�Dead Man Walking' mission. Once you have activated the 'Locate the Nanosuit Cradle' objective, head into the elevator. When the elevator reaches the bottom, go to the right. At the end, don't turn right, but press action on the power box. This will show a red light. Backtrack to the first elevator, but jump over the red velvet rope and go down the corridor past it. Activate the switch on the wall to the left, and an elevator will come up next to it, showing the humorous Easter Egg.
Gatekeepers Easter Eggs:
On the Gatekeeps mission, having passed the CELL checkpoint, head up the stairs to the left on the approach to the building's entrance. In a bush to the left are three actual Easter Eggs.