The hidden star of week 3 is waiting for you if you have completed at least 3 challenges of The Leftovers Mission. Once the secret loading screen is unlocked, look at the bottom left of the image just next to the headlight.
You can see a sticker representing the ice store just below the circuit in the desert.
Go there and enter through the main door. The hidden star awaits you on a counter at the back right of the store.
