Did you start the Shootout at Sundown challenges? Did you complete at least 3 of them and unlock the secret loading screen? Perfect, you can now look carefully at the loading screen: at the top left — inscribed on the barn wall — a knife and a fork are engraved.
Go north of Fatal Fields to the knife and fork shaped quarry.
Stand between the knife and the fork, on the ground that is not covered with grass and the star will be waiting for you.
You can still pick it up after the missions have disappeared, as long as you have unlocked the secret loading screen.
The clue to unlocking the hidden Battle Star is in the B.R.U.T.E Squad loading screen.
