On my third character, I was ready to fight at least three bandits outside the Ustengrav entrance. When I cautiously approached and found the usual three bandits dead. No experience to gain here. I usually find one dead bandit in the pit next to the door where the skooma is. He was still there, but the other three I usally fight were pre-killed. It was daytime and I did not see what could have killed all three of them them. I was at level 13 or 14. I was in a hurry anyway so I did not mind missing out on casting some destruction and conjuation spells.

Could have been anything. There is usually a necromancer there as well.

There is also a Chaurus Reaper that spawns in the marsh nearby.