Jump from the opening at the end of the hallway with the spotlight and machine gun to a ledge on the outside of the other building. Loot the bodies of the 2 fallen enemies and pick up more arrows if you need them.
Turn left and go all the way around to the other side of the building.
There you'll find a food cache and a GPS cache (2/2). Then go back around to the corner where you jumped over from the larger building.
NOTE: If you're close to having enough salvage for another weapon upgrade but haven't quite got enough, you can drop down, head back toward the base camp, and hunt a few rats. Each one yields 15 XP and a few salvage points. Use the camp to make your desired upgrade. Then return to the building, climb the stairs to the upper level and jump across to the second building again.
Scramble up the wooden wall and follow the walkway around to the top.
Pick up the pistol ammo and use the zip line to slide into the next area.