Ive played near on 100 hours and have never seen an NPC steal anything? does it even happen? In Oblivion you'd get the odd one sneaking about trying to theive food, has this been taken out? I have seen one NPC murder another but that started off a quest so i reckon that was a scripted event. And i'm not taking about faction killings like bandits forsworn etc cause thats bound to happen as people migrate.

Happens constantly in Riften. I even saw the guards fighting thieves and merchants fighting guards and Riften Guards vs Imperials all at once. It was amazing.

There are NPC Thieves in the game. You may see them while you are doing certain things.

As for stealing from you, rather your home, I don't think that happens. I did at one point, but it was just a sword that fell into the void from a rack. It was still in my inventory, as far as the quest giver was concerned.

However if you put stuff in containers that aren't yours they can vanish. Question is, do NPC's take them or does the game just do away with them?

Riften is an exciting town. Just drop something on the street and watch them all fight over it. Even Mjoll, the city protector will kill over a dropped iron dagger.

It would be nice if the other members of the guilds in this game went out into the wide world to help their guild too, imagine being asked to accompany the thieve guild members to help them theive etc so like all 5 of you go to a town and steal stuff cause it seems to me that all they ever do is wander the guild hall and do nothing to contribute.