There are a total of 4 Useable Vehicle Keys in The Walking Dead: Survival Instinct. Finding all of them will earn you the “No Down Payments” trophy / achievement. If you die before reaching the next checkpoint you have to collect the last key again. They are not saved immediately.
WARNING: You cannot get all collectibles in one playthrough! You have to play through the story at least two times. At some points you can travel in two directions with your car. If you choose one location you won’t be able to visit the other unless you start a new game and replay the areas. All statistics (collectibles, kills etc.) will stay if you start a new game in the main menu. Choose all the locations on the right side during your first playthrough and choose all the ones on the left in your second playthrough.
#1 – Fontana
After helping Mia in the theatre, go back to the restaurant where you found Scout earlier. Scout and her friend will now be gone, but they left a letter and useable vehicle keys for you. The keys are on the kitchen table, at the exact same spot where Scout was standing when you first met her.
#2 – Lemon Hill
When you search for the fireworks you will come to a dark room on the rooftops. There is a ladder going down to a yellow bus. Before you go down that ladder, look for the key on a bedside cabinet, to the right of the window where the ladder is.
#3 – Archer Creek
When you enter the area where snipers cover you, go into the building that has a big white sign saying “Archer Creek Utility” above its door. Search one of the side rooms (you need to open a wooden door) and look on the shelves to find this vehicle key.
#4 – Lafferty
After you push the car to jump over the fence, you will have to go through a very dark shop. At the end of the shop, a walker will break through a wooden door. The last vehicle key is in the small side room where the walker came from.