Prepper Stash: Long Range Lockpick
As the name may imply, you'll want to have a good long-range sniper rifle along for this mission. Find it at the Deep North Irrigation Reservoir, northeast of Fall's End.
The starter note is on the shoreside of a walkway which leads out to a small observation station over the water.
The stash is in the station, but you can't break in from this side. As a note on the door suggests, you'll need to shoot the lock through a window on the back wall.
Climb up on the station roof, and zipline across the water to a small house.
From the house roof, jump left (west), to a sniper platform.
Take aim at the station. You need to hit the door lock, but you'll know when you do- sirens will start going off and your marker will turn into a GET goal.
Swim back, or otherwise traverse the water. Cultists may have been drawn by the ruckus, but if you move quickly you can slip past them, or just pick them off. When ready, enter the station.
Loot the station to complete the mission.