This secret challenge came with Fortnite update 12.30. It most certainly announces a broader event that will take place in the coming weeks. It's called The Gnome Who Saw the Bear and consists of finding garden gnomes near telescopes to point them in a certain direction. The bear refers here to the constellations of stars.
Completing this challenge will reward you with 40,000 XP, same as for a classic challenge.
Map
As you can see, all the telescopes to be activated near the gnomes are located within a small radius, northwest of Weeping Woods.
It remains to be seen whether other secret challenges will be added to the game or not. This is still a good way to raise your battle pass level without forcing yourself too much!
