I assume many of us had this.

You walk the streets, and you find a corpse of a person, not knowing what killed him or her.

This happened with my current character, Galadri?l, a graceful wood elf who will one day become the Guardian Of Auri-El. She walked in Windhelm and found Angrenor. Unfortunately, lifeless.

I don't know what kiled him, maybe a bug. Anyway, the man's a beggar. So I roleplayed that the harsh cold and the hunger became too much for him and his war wound.

"As the young elf walked the streets, wearing thick fur against the harsh cold, she found a beggar. Shocked by his state, she ran to the man in the hopes of helping him. Unfortunately, his heart was not beating...the hunger and the cold became too much."

Guess you can always make such things a part of a story.

Oblivion was known for killing off NPCs when you were not around, it does not happen as much in this game, but it can still happen.

Vampire attacks.

In windhelm the vampires can spawn at the main gate and behind candlehearth hall.

Is that so? I have played Oblivion but I never knew about this.

The more you learn, I guess.

It's likely a vampire attack that occurred too far away for you to notice it.

Oblivion has a feature where NPCs travel around doing their own things. Well, the world is a dangerous place and these NPCs can die when they are out and about. Or, they fall off a bridge (although I think that one was patched as it was an important NPC)

As the others said, likely a vampire attack. Did the guards give a hint?

Try Stump! I was strolling through Riverwood the once, and he just died. Literally, he just died and I have little idea what could have been the cause. The only possible explanation was a chicken, but is there any reason for one to attack Stump and for him to die in an instant without putting up any fight?

In Skyrim, the main cause of NPC death is Vampire Attacks, post-Dawnguard.

Coincidentally, that's also another reason not to run a "No Essential NPC" mod in Skyrim.

Markarth was notorious in my games for the numbers of character deaths that happened there! Which makes sense--it's a dangerous place to walk, even sober, in full daylight!

In Fallout 3, NPCs in Megaton sometimes had a habit of falling off the raised walkways to their deaths as well.

This reminds me of those NPCs in San Andreas who snap a picture at the side of a road in the countryside just to walk until they fall in the nearby lake to their deaths. Though that's an Easter egg.

Amren followed me into Breezehome one day, and then left. I stepped out after him, only to meet a courier with an inheritance letter stating Amren had died. His body wasn't anywhere around.

I've found people mysteriously dead without Dawnguard. I think the game engine sometimes generates them up in the air and they fall to their deaths.

Death Hounds are even worse than vampires, in my experience. They are known to just rush in, slaughter stable hands, then dissapear.

Do you have the kill upon arrow to the knee mod? I started finding dead NPC's only after putting in this mod.