You've clearly never been attacked by ruffians while traveling unitless.
It doesn't take mobs: it takes a few highwaymen with daggers.
I suppose I should clarify:
IMHO, it is metagaming to have your character act with certainty that a noble (a noble!) slunk around in the shadows with a dagger. Far more likely to be one of those darn adventurers.
Many, many, many assassinations happen without an obvious culprit: just because you've never seen a person other people thought was a commoner (my characters have often doubted the lineage of other "nobles") kill someone doesn't mean you would necessarily think it incredible.
