The secret Discovery Banner the second week to Fortnite Battle Royale Season 8 is located at the volcano, southwest of Sunny Steps.
The location of the secret banner can also be seen in the unlocked loading screen, to the left of the door is the hidden icon.
To actually make the banner appear, you must first complete a full set of weekly challenges (weeks 1-10).
Once you have completed a set of these, this secret banner can be unlocked and the challenge “Find the Secret Banner in Loading Screen 2” is displayed in the Explorer Challenges. It will reward you with a full battle pass level.
An overview of all the challenges of Week 2 in Season 8 can be found here.