High Plains Drifter

 refers to an unmarked quest in Fallout 3.

On the top floor of Dickerson Tabernacle Chapel is a sniper called Drifter. He carries the Reservist's Rifle as well as the holotape for Oasis. A random glitch can make Drifter seemingly disappear, never to be found again.

He is always hostile to the player and will attack the player once they reach the building. As he is on an unreachable ledge, he must be dispatched with a ranged weapon or grenades. Sometimes he might drop from the ledge to engage the player, allowing for more options to kill him. The real challenge is to kill him in a way that puts him in a lootable position as you cannot reach the planks he is on normally.

Ways of getting his corpse to fall down

 * The easiest way to loot drifter is to shoot off the nearest available body part so it falls off the platform.
 * Another easy method is to wait and watch him from a distance until he moves to stand in front of the window, then snipe him in the back so he falls through it.
 * If Drifter is stuck on the upper floor of the Chapel, and you can't reach him to loot, fast-traveling to a remote location and returning, may result in him miraculously falling down to loot.
 * If you have a missile launcher you can fire one at him and the explosion should knock him down so you can loot him.
 * You can also simply throw a grenade up there and blow his body off of the ledge.
 * Additionally, there is an easier way to come in reach of Drifter's platform if you are playing the PC version. In the rear of the church is a soap box containing two Stimpacks and a box of laundry detergent; simply move this box directly underneath the platform.  From this point, you should be able to stand on the box and jump just high enough to loot Drifter's corpse.
 * In the PC version, it is possible to reach the ledge by turning off collision and simply floating/flying to his location (press ~ then type tcl into the console).
 * One more way to possibly loot the drifters corpse, blow up the two cars in front of the chapel, this will occasionally knock the corpse off the ledge.
 * If you don't want to deal with trying to get the Corpse, aim to shoot the Rifle out of his hands, allowing you to simply walk over to it and pick it up. He also has no other weapons he can use, so he's harmless without his rifle. (Though easier than many other strategies, you also have to get the weapon repaired after this, as it is usually broken.)
 * In this PC User's run-through, the Drifter simply came down from the ledge to engage a randomly-spawned Giant Radscorpion, which killed him. The corpse is then normally lootable.
 * It is possible, with some difficulty, to jump on the lectern below the platform and then on the diagonal planking which covers one of the windows, then turn to the left and walk slowly forwards, gaining height until the player is close enough to loot the corpse.

Reward

 * Reservist's Rifle
 * Oasis location