Thieves

You can adjust the spawn chance (0% <-> 100% -- vanilla is 21%).

You can enable or disable the events in Markarth, Riften, Riverwood, Solitude, Whiterun, Windhelm, Dawnstar, Falkreath, Morthal, Winterhold.

The MCM menu defaults to everything disabled.

You need to set your preferences or there won't be thieves anywhere.

I've also made a few changes to the original quest to suit my personal taste.

Vanilla

The thieves are freelance (well, the thief... it's always the same one, possibly resurrected by Nocturnal).

They always try to burgle the same place and if the owner happens to pass by, he completely ignores them.

If you go near one of them without attacking, the dude usually leaves the door and wanders aimlessly around.

If you or a guard attack them, they become hostile to everyone and the entire town joins the fight.

They usually die, but they might also enter a building, in which case everyone is stuck in combat mode unless you run after them (possibly trespassing and getting fined).

The quest doesn't always remove the thief when stopped.

You might have previously-undetected thieves wandering around without AI and possibly attacked by the guards while doing nothing bad.

You might even find more than one thief, one lockpicking and others strolling the streets.

Modded

The thieves are randomly taken from your bandits' leveled lists and are considered new Guild recruits.

They wear TG armor and appear when the Guild begins to recover from its misfortunes.

Each city has a few randomly-chosen targets so that the thieves aren't always in the same place.

If you ignore them, they ignore you.

You can "Ahem!" them (if you're not a guild member yourself) and they run away... better safe than sorry.

You can always attack them, of course.

If you're Guildmaster and go near them, they recognize you and might say something appropriate (not much).

If a guard (or a guard-like NPC) attacks them, they try to escape while the common citizens keep minding their business.

If you keep ignoring them, you're not forced into combat state (red dot) and can go on with your business.

If you hit them, they get really pissed off and accept the fight, Guildmaster or not. In this case citizens and followers join in.

If you instead decide to defend your fellow Guild members by fighting the guards... well, that's up to you, but it could have consequences.

They're cowardly and weak (30% PC level), and they try to keep running ("Stop, thief!")

If they enter a building, they manage to escape (back door, back window, or back whatever) and the quest stops.

The quest always deletes the thief when stop