I did some searches and there a few threads months old, but none really seem to capture this idea, so...
In an effort to further complicate established fortress life....The Thief.
The Thief would be a secret job that is assigned to one of your non-legendary dwarves once your wealth/population reaches a certain level. The Thief would also require the dwarven economy to be active.
While "working" the thief would read as "on break." They would regularly walk into people's rooms and take items from them. The owner gets an unhappy thought when the announcement "Urist McLandowner has discovered +Goblin Bone Earrings+ missing from his room!" shows up.
The Thief would store these items in secret on their person (Yes, that's a little out there if they've been active for a long time, and breaks the encumbrance limit rules, but it makes more sense than them storing it in their room...or creating containers that you would eventually discover.)
The Thief can only be uncovered by the Captain of the Guard and members of the Fortress guard. It would be a proximity-based test relying on the Judge of Intent skill. (Or perhaps some others.)
Once revealed, the thief stops being a member of your fortress and is treated as a hostile, and flees toward the nearest exit. This is a natural reaction, since the punishment for stealing from the fortress is death anyways. If you choose to imprison the thief by using cage traps or whatever, you'll have to contend with their superior restraint and door opening skills. (Perhaps the Thief has a secret job level that impacts these tests.) Best just to cut the bastards hands off and throw him into magma.
When the thief is revealed, they drop all the stolen loot they have, and the owners of recovered loot all get a big happy thought for seeing justice done. There would an interval period between the discovering of one thief and the activation of another.
Some caveats-
-Nobles cannot be chosen as the thief. They own everything, after all.
-At the time of Thief selection, any active military dwarves are excluded.
-Members of the Fortress Guard are never chosen as the thief.
-Legendary dwarves cannot be chosen as the thief. Thieves that become legendary stop being thieves, a new thief is selected, and the loot is placed in their inventory. That way it's like the original thief was never really active. The same applies to thieves who are moved into the fortress guard.
-Thieves do not steal, and cannot be revealed, while they are active members of the military who are on duty. This would prevent players from systematically stationing dwarves next to the Cpt. of the Guard to try and reveal the thief.
-Thieves that are part of the military DO steal when they are off duty, and can be revealed then.
-Thieves who are killed by something else (magma, goblins, HFS, ect...) would inadvertently be revealed when they die and their corpse is sitting on top of a mountain of loot.
While it would be cool to have a whole thieves guild running in the fortress, because you need to punish thieves and they would ultimately become an enemy of the fortress, you don't want to tie up a lot of the population doing it. And people will already be >< enough when a promising dwarf they were looking at turns out to be a klepto.
This might also encourage some fortress owners to take better care of their dwarves.....especially if an artifact has gone missing. Would be a shame to have that new peasant miner go dig out your magma channel...and watch some valuable stuff go up in flames.
There could be some additional traits that affect thief selection, such as:
-Personality types
-Room quality
-Average seasonal income
-# of owned items
-Raw agility
-Deity (God of Thieves?)
-Highest job skill (The least skilled job being the most likely to take up thievery)
Traits could also affect who the thief decides to steal from/what they steal:
-Dislikes authority (Wealthier citizens)
-Doesn't care about others (Anyone, or particularly the poor)
-Confident (Goes after high priced items, artifacts, the King's Throne, ect...)
-Doesn't do more than is necessary (prefers cheap, easy to get items)
-Other traits that affect how often the thief decides to steal.
And on.
There would still be ways to try and corner the thief...but they would be time-consuming (rotating people in and out of the Guard and military to zero in on the thief) and would disrupt work flow, so obsessively trying to locate the thief would come with it's own penalties.