The secret Discovery Battlestar of the first week of Season 8 is located on the tower next to the ship in Lazy Lagoon.
To actually make the battlestar appear, you must first complete a full set of weekly challenges (weeks 1-10).
Once you’ve completed a set of them, this secret battlestar can be picked up and the challenge “Find the Secret Battlestar in Loading Screen 1” is displayed in the Discovery Challenges. It will reward you with a full battle pass level.