Reaver's Canyon
A bandit camp northeast of Klysma Quarry. You'll come through here during the Ballad of Si-Mut and Gertha.
The local treasure is in a tent on the east side of camp. Surprisingly, it's completely unguarded for much of the day.
Drop down and loot the red chest in the corner to claim it.
That just leaves the captain. If you don't want to get into a fight with everyone in camp (and no one's judging you either way), just snipe or dropkill him, and walk out.
Either way, his death completes the area.
Note that your scans may show a treasure to the south by south west. There is one here, guarded by a lone but tougher-than-average bandit. It doesn't count toward any location completion, however.