Your foresight has served you well: on finding notes in Strelok's group’s stashes, you kept them until you found their rightful owner.
The player can acquire this achievement by delivering all three of Strelok's notes to him before the final Evacuation. The notes are found in Yanov and can be acquired any point in the game, though their location can only be acquired by completing the UAV side quest.
The three notes are contained in three very well hidden stashes in Yanov in: The Quarry, Cement factory and the Jupiter plant.
The stash in the Quarry is in the cabin of the Bucket wheel excavator which cannot be accessed by normal means. To get to it, go to the highest point of the hill overlooking the quarry, where one will see a dead tree. Climb onto the cabin using the tree and search the cabin for a black backpack.
Note that the stash ISN'T in the actual plant but under it. To get it: circle the cement plant until finding an underground entrance just to the north next to the bridge, and inside it the player should see a hallway with a broken gate. The stash is located in the green crate.
The note in the Jupiter factory is located within the main building grounds, not outside it. It's located under a row of watertowers and stashed inside a bellmouth pipe.
- Strelok gains increased damage resistance during the finale.
- Strelok gives a flame, firefly, and three scientific medkits or two batteries (20 rounds) if Degtyarev has a gauss rifle in his inventory.