Prepper Stash: Dead Man's Treasure
The Dead Man's Treasure stash is at Dead Man's Mill, northwest of the Moonflower Trailer Park. This is a grapple-required stash.
The note is out front (west) of the cabin atop the falls.
Enter the cabin by going around to the back. You'll actually come up through the floor.
Grab the yellow gear inside.
Follow your marker to a small metal peg, then attach the gear, and interact with it so stop the waterfall.
Look down for a grapple point.
Lower yourself just a little to see a narrow cave in the rock face.
Enter the cave and follow it in to find the stash and complete the location.