The secret Discovery Battlestar of the first week of Season 9 can be found just south of Lonely Lodge.
To actually make the battlestar appear, you must first complete a the full set of weekly challenges.
Once any four sets of them have been completed, this secret Battlestar can be collected and the challenge “Find the Secret Battlestar in Loading Screen #1” will appear in the Discovery Challenges. It will reward you with one full battle pass tier.
An overview of all challenges of week 1 in season 9 can be found here.