This challenge is part of the Brutus' Briefing, the first agent on which Fortnite's Chapter 2 Season 2 focuses. You'll have to to find a Shadow safe house of your choice. These lairs are accessible through hidden passages, in the purest respect of secret agent films. On the map below, you'll find the locations of the hiding places that will propel you into the Shadow safe houses.
Map
East of Pleasant Park
Shadow's first safe house is under Pleasant Park. You can access it by entering the toilets located behind the gas station. Posters making the apology of the Shadow gang decorate the building!
East of Frenzy Farm
The second safe house is located on the outskirts of Frenzy Farm, in the small brick annex of the petrol station.
In both cases, the safe houses are populated by heavily armed Shadow henchmen. Be sure to retrieve at least one means of defense before venturing into them.
Finally, use the Solo and Duo modes to complete these challenges.