DdraigGwyn
Cymru
It's just simple game mechanics. If you attacked and killed them and it wasn't in self-defense, it's murder. If there are no witnesses left after you kill them then the crime is removed from your status. Unless you kill all the bandits in Fort Greymoor you are guaranteed to have surviving witnesses to your crime. Even if you kill all the bandits as well, if the fight carries over too far to the east the guards at the Western Watchtower may become witnesses as well.
The last option is the only one I can imagine: I had cleaned out Greymoor and killed all of the Thalmor. Must be those eagle-eyed guards. Memo to self: next time, kill all the guards as well.