The game just fucking trolled me!
I'm playing Skyrim in survival mode, and I'm walking from Winterhold to Whiterun. Right as I was approaching Tumble Arch Pass, I was unable to sneak, no matter where I moved. Not even the invisibility spell helped. So I found the faint red dot of the random enemy that locked on to me.
It was a random goddamn troll! And to make matters worse, as soon as I managed to kill the troll, another one came up from behind!
I once walked from Riften almost to solitude (the marshy land to the south) because a random slaughterfish had aggro'd without me noticing and the game still considered it a valid enemy.
Half an hour of running toward a dot later I finally found it...
I cast fireball and pretended it was a D&D fireball.