The Secret Shootout at Sundown Battlestar in Week 2 of Season 10, can be found by the Fork Knife cave next to Fatal Fields.
To actually make the battlestar appear, you must complete 3 of the Limited Time missions in the Shootout at Sundown Challenges.
Once you’ve completed 3 of the 7 challenges in the Shootout at Sundown missions, you can collect this secret battleship. It rewards you with a Battle Star (a full Battle Pass level) that increases your X Season Battle Pass.