The secret Utopia Battlestar of the fifth week of Season 9 can be found just outside of the castle next to Haunted Hills.
To actually make the battlestar appear, you must first complete a the full set of weekly challenges.
Once any five sets of them have been completed, this Secret Battle Star can be collected and the challenge “Find the Secret Battlestar in Loading Screen #5” will appear in the Utopia Challenges. It will reward you with one full battle pass tier.
Here’s the hint: This time the hint is pretty obvious. Right in the picture you can see 4 coordinates. They are “B2, B3, C2, C3”.