Fallout 3 random encounters

Throughout the world of Fallout 3, you can encounter random events in the Wastelands. There are certain places where random encounters can happen, but once "activated", a random encounter in a particular area won't change.

You can save outside the spawning area and keep reloading if you don't get the encounter you want.

I've also discovered that once you find the UFO encounter once, you can't find it again. I've tried for several hours at locations that the UFO encounter appears at and it wouldn't spawn after I've already found it.

Random Encounter Locations
These are the known locations so far:

Northwest Quadrant
 * In front of MDPL-05 Power Station.
 * Behind the ruined house on the road to Germantown, near Paradise Falls.
 * Rockbreaker's Last Gas.
 * North of MDPL-21 Power Station, in a pathway between the rock hills there
 * Outside of WKML Broadcast Station.

Northeast Quadrant
 * North west of the Clifftop Shacks (approximately half-way between the Clifftop Shacks and Oasis on the world map), near an irradiated pool.
 * Meresti Trainyard.
 * Halfway between Meresti Trainyard and Minefield
 * In front of the Robot Repair Center, south of Canterbury Commons.
 * In the Temple of the Union, but only if the slaves have been moved to the Lincoln Memorial.
 * At Relay Tower KX-B8-11.

Southwest Quadrant
 * Vault 106
 * North of Smith Casey's Garage, by the army camp.
 * In front of Rockopolis.

Southeast Quadrant
 * Barn behind Vault 101 entrance/exit.
 * On the second road north west of the Super-Duper Mart.
 * In front of the Super-Duper Mart (but its only worked for me when I came from the rear of it).
 * In front of Alexandria Arms.
 * At the entrance to the Flooded Metro, west of Arlington Library and west of a raider camp
 * Anchorage War Memorial

Types of Random Encounters
The random encounters have been informally divided into four-types. All encounters have the potential to become violent if the player chooses to attack the NPC. However, for the purposes of this classification, violent or potentially violent encounters are those in which combat cannot be avoided; or can become violent through a dialog choice.

Non-violent encounters involving at least one humanoid
 * A Wasteland Doctor.
 * A Wasteland Junkie.
 * A Scavenger with one or more of the following pets: a Brahmin, Dog or trained Yao Guai. He'll offer you a Nuka-Cola Quantum.  You can barter with him into reducing the price.
 * Two Wasteland Settlers and two Escaped Slaves fighting over a refrigerator of 10 Purified Water.
 * Two Wastelanders and an Enclave Eyebot.
 * Three Hunters.
 * A lone Hunter selling meat.
 * Uncle Leo, a friendly Super Mutant. If you tell him you're going to rob him, he'll give you a Dirty Pre-War Businesswear as a present. If you attack him he'll run away instead of fighting.
 * A Wasteland Merchant. After having a talk with him about the Great One, he'll collapse and die.  You'll find a Wasteland Merchant Note on his body about him going to Oasis the next day.
 * You'll receive a Chinese Radio Beacon over your Pip-Boy. You'll find a Dead Chinese Commando at the spawning point.
 * Two Frag Mines and two Wastelanders, one of whom attempts to disarm them (unsuccessfully).
 * A Scavenger with his malfunctioned Mister Handy. If you repair it, the Scavenger will give you two Energy Cells.
 * Ghoul Wastelanders around a burning barrel.
 * Three Slavers with three Slaves.
 * A man named Ben Canning. He asks for purified water.
 * A Wastelander killing and looting a molerat (only after completing the Wasteland Survival Guide quest). He has different dialogue options, depending on whether or not the player completed all bonus objectives given by  Moira. If the player says he has helped with the book, a congratulatory or a mocking response is received. If the player says he has never heard of the guide, the Wastelander will provide him with a copy of the book.

Violent or potentially violent encounters involving at least one humanoid
 * A Talon Company ambush if you have a Karma rating of Good.
 * A Regulator ambush if you have a Karma rating of Evil.
 * A Hitman ambush if you have a Karma rating of Neutral.
 * Two Hunters attacking a Giant Worker Ant.
 * Three Hunters attacking a Vicious Dog.
 * A Deathclaw and a dead Wastelander. You can find Deathclaw Gauntlet schematics for the Deathclaw Gauntlet on the dead Wastelander.
 * Mel, a modern Highwayman who will demand caps and items from you. You can either utilize speech options, or kill him.
 * Laszlo Radford (He's hostile and will open fire on you with his Railway Rifle).
 * Sam Warrick shooting at a Wastelander (if he kills the Wastelander, he'll turn on you).
 * Five Raiders, four beating the fifth one from all sides, in turns. It seems to be an initiation.
 * Three Raiders and a Wastelander with a time-bomb on him. You can disarm the time-bomb if you have a high enough explosives skill.
 * Three Wastelanders with info about Oasis. If you ask them about it they'll turn hostile.
 * A Scavenger and his dog fighting a Protectron, Robobrain or Super Mutant. If you go near the Protectron/Robobrain/Super Mutant after you kill it and the Scavenger is still alive, he'll attack you.
 * Mole Rats attacking a Wastelander (if you save her she'll give you a small amount of caps).
 * Radscorpions attacking a Wastelander.
 * Two Wastelanders fighting a Giant Radscorpion over a fridge of 8 purified water.
 * Vicious Dogs attacking a Raider or Wastelander.
 * One or two Hunters fighting a Yao Guai.
 * 3 Hunters attacking a Giant Soldier Ant.
 * If you have not recruited Dogmeat, it is possible he will appear during random encounters to come to your aid.
 * Two Brotherhood Outcasts and a Protectron.
 * A small squad of Brotherhood Outcasts being chased by two Deathclaws.
 * Three Wastelanders fighting with a Deathclaw. If you save them they'll give you the location of Rock Creek Caverns.
 * Two Slavers who will shoot you if you fail the speech challenge.
 * A Vicious Dog, two dead Vicious Dogs, and a dead Wastelander. On the Wastelander's body is a key and a note telling you he apologized to his brother and where his treasure is in Scrapyard.  In the box are three skill books.
 * Three Megaton refugees who attack you with knives (available if Megaton is destroyed)

Non-violent encounters involving only creatures/non-humanoids
 * A UFO exploding overhead, spilling 24 Alien Power Cells and a Firelance all over the area.
 * A Vertibird getting hit by a missile and crashing near you.

Violent or potentially violent encounters involving only creatures/non-humanoids
 * Two or three Mad Brahmin
 * One or more Giant Ants fighting a Giant Radscorpion.
 * Several Giant Ants around a toxic barrel.