eh so? the whole point about thief/assassin is: noone notices you EVER.
as assassin... you should wait till everyone is asleep or just one there, kill and dispose the corpse. then move on to the next guy, when a patrol guy comes around the corner and sees the blood, ok now he wonder, but the assassins just kills and goes on again. the same applies to a thief, but instead of killing he steals.
what I talk about is when you already blow your cover. when the assassin attacks and DO NOT kill... shall the victim forget about the attack just few seconds later? no. Its ridiculous... the "bandits" see their dead comrade, they see the arrow in the neck, the lots of blood and they just go "ah poor bob, he always were sucidal"... yes you should be able to go into hiding again when you just were "too loud"... but its completely different when a lot of blood is the proof of your presence.
I was really hoping for a few quests like that: You must infiltrate a heavily guarded place, and kill guards patrolling the area (or sneak around them) as you slowly advance to your main target.
And as a few others have stated, the Hitman games have decent AI; they actually react to finding dead bodies and stay on alert for quite some time (sometimes for the rest of the mission). Of course those games are dated, and the AI can still be easily manipulated, but I don't see why Bethesda wouldn't be able to improve AI in that area: to make them be alerted for several minutes after hearing a noise, getting hurt by the player, or finding a dead body.