Loot the bodies here. Then move around the hole in the floor and jump across the gap above the horizontal pole to reach the salvage crate and food cache in the corner. Turn left and walk across the metal pipe to the next corner.
Here you'll find another food cache, a salvage trunk, and another flag (2/4) that you can destroy with a fire arrow for the Previous Inhabitants Challenge. It's hanging on the south wall just beyond the food cache. It's partially obscured by greenery, and the square column can block your view of it. Use Survival Instinct if necessary to spot it.
Now make your way back across the pipe, across the gap, and around to the right to the far end of the walkway.
Move to the top of the ramp made out of a metal door and jump to grab the dangling platform.
Traverse around to the right and pull up.
Move to the top of the sloping platform and jump to grab the edge of the roof ahead.
Pull up and turn around to find the next flag (3/4) for the Challenge. Burn it with a flaming arrow or torch. Just watch out for the holes in the floor as you explore here. Raid the food cache near where the flag was hanging and loot the bodies of the men who fell here.
Make your way over to the western corner of the roof to find a GPS cache (3/5) hidden in the grass near the broken column.
Move to the edge of the roof near the rope-wrapped thingummy and look up to the left (east). On the side of the tall tower is the final flag (4/4) for the Challenge.
It's pretty high up, but you can hit it with a fire arrow from here. If you've burned the others as well, you'll receive 150 XP.