Once you’ve completed the challenges of the current Fortnite Battle Royale week you can uncover the secret banner, in week 2 of Season 7 there is no hidden battlestar.
If you find the banner you can use it as a profile picture in your account.
An overview of all the challenges of the second week can be found here.
Where is the secret banner hidden in week 2?
If you have completed all the challenges, you will see a different picture in your loading screen. You recognize the heroine Lynx, she sits at a desk and is probably working on a new suit.
With this note, you should go to Frosty Flights, in the large building you will find the office of Lynx. In the video below you can see the exact location.