Thieves are similar to bandits in a few ways and like raiders in a few others, but they are mostly unique since they usually operate inside of settled areas, and especially densely populated cities. Like bandits they will sometimes ambush the unwary in alleyways or at blind corners, and like raiders they will often try to overcome the security of target locations, though typically by stealth, cleverness, or acrobatics rather than main force as with raiders. Small groups of independent thieves usually have motives basically the same as bandits. Thieves’ guilds, however, tend to be more ambitious and have complex goals that their activities mostly serve to fund.
Reasons for Thieves to Take Captives
Nerves. Like bandits, thieves generally intent on murdering anybody, as that will bring a lot of heat down on their thieving crew from the much nearer authorities. People winding up dead will cause an outcry and force the guard to employ a great deal more manpower to address the problem than thieves who conk a few doormen on the head and flee with a painting or sack of gems.
Get People Out of the Way. The stereotypical reason thieves will capture someone and keep them tied up or locked away somewhere is to prevent them from getting in the way of a job. This can be before the heist, in order to prevent complications, or during a job when someone needs to be subdued and then whisked out of sight so no one else notices the unconscious people and sounds an alarm.
To Take Their Time. It is far less likely for thieves to get caught searching through their prey’s pockets if they are inside an abandoned building somewhere than still in the alley where anyone might happen to walk past or respond to shouts for help.
To Steal Credentials/Access. More elaborate thefts might require obtaining guard uniforms, keys, and other items in order to get to where they aim to go and to blend in when they get there. Usually these items are kept on someone’s person, or would be reported missing if the individual they are taken from is free to report back.
Kidnapping. Particularly ambitious thieves or guilds that deal in many sorts of organized crime might dabble in kidnapping in exchange for ransom, either demands to be fulfilled or money to be paid for their release. Obviously a ransom play wouldn’t work out very well for them if their victim is wandering about the city in public.
A Diversion. Thieves’ guilds might send out crews to perform kidnappings of people who will be missed and send ransom demands simply in order to keep authorities and pesky bounty hunters and adventurers busy while the real job happens somewhere else.
Slavers. Extremely heinous thieves may make deals with unscrupulous slave traders (and smugglers if the slave trade is not legal in the city where they operate), and view particularly in-demand sorts of people as simply another piece of loot to steal and fence.
How Thieves Take Captives
Intimidation. Muggers and other particularly threatening sorts of thieves generally prefer to simply brandish weapons and order their prey to tie their own hands and walk into some out-of-the-way place under their own power. This is especially effective in cities or districts where people are not normally permitted to carry weapons openly, as it minimizes the risk of their victims fighting back.
Force. Catching people off-guard with a flurry of swings with a blackjack or truncheon is often an effective way to knock victims unconscious before they can put up too much of a fight. This is also the ideal backup plan when threats fail to persuade a victim to cooperate.
Traps. Nets, snares, trap doors, piles of refuse that can be quickly toppled to block an escape route, one-way doors, slippery ramps, knockout darts, sleep gas, paralysis or suggestion glyphs… anything that could work as a non-lethal dungeon trap could be set up in an out-of-the-way urban area by those with the knowhow.
Ambush. One of the more common methods of swiftly handling victims is to find some way to lure them into a location where the thief’s allies lie in wait to employ any of the above methods abruptly and in force. Common bait strategies include an urchin or other pickpocket intentionally being noticed as they flee down an alley with a bit of the target’s possessions, a distraught-looking person begging for assistance leading well-meaning sorts into the trap, or passing rumors around in treasure-hunting circles about some hidden cache in the ambush area or somewhere that can only be accessed by passing through it.
Subterfuge. Knockout poison and the like don’t necessarily need to be delivered by blowguns or crossbows. They can also be slipped into drinks, burned in incense form and wafted through windows, carefully applied as lipstick over a thin layer of wax, or delivered by a ring with a hidden needle. There are many ways well equipped thieves with better social skills than combat abilities could render a foe unconscious or docile.
Seduction. For all of recorded history, one of the easiest known ways to get someone to lower their guard is to have an individual they’re attracted to lay on the charm. This could leave the target open to being drugged or unarmed and unarmored when a brutish visitor comes and subdues them. It could also simply involve tying them up for what seems to be more recreational reasons, before they realize anything is amiss. If played right, they may never even realize they were being kept out of the way at all.
Waiting. Often thieves simply want those they capture to be out of the equation for a few hours, a few days, or a few weeks, and have no reason to keep them captive once the job is complete. Or maybe the place they are keeping their captives isn’t very hidden or might even not be under their control and someone will eventually stumble across the victims and free them.
Cooperation. If their goal is to rob or mug rather than perform a bigger heist, thieves may let cooperative victims leave (minus their valuables of course) or leave them trapped in a way that will only hold the victims for long enough that the thieves can make their getaway. Afterwards, enterprising adventurers so waylaid will almost certainly find a way to resupply and then track down the malefactors to reacquire their lost possessions.
Bargaining. As with bandits, a silver-tongued adventurer might be able to offer something more valuable to the thieves than what the thieves thought they were getting by capturing them. Whether the offer is real is largely immaterial, though bluffing will probably create a whole new risk of being recaptured.
Observation. Since thieves generally rely on disused parts of a city to avoid the attention of the guard, the places they keep captives are likely either in disrepair or have long-forgotten access points that even the thieves may not have discovered. Careful investigation of the party’s surroundings can often uncover some sort of path either to freedom or—unbeknownst to them—a different, greater danger.
Infighting. Again like bandits, thieves are often not working together entirely out of their own free will, and while necessity or greed may make them work together, it is not the strongest of social glue. Manipulative captives might turn them against each other, and possibly even ingratiate the winners, leading to their release.
Intimidation. Thieves are in it for the money, or maybe the challenge of stealing. They aren’t in it for a trip to Valhalla. Putting up a strong enough resistance or threatening them with what will happen to them later if they don’t let you go will often change their tune. Especially if the party promises to prevent the consequences they fear if they cooperate right away.
What’s in it for the Party?
Disrupting the Heist. If the aims of the thieves are directly opposed to the party, or if they both seek the same macguffin, the most important reward might be interfering with the thieves’ job. With luck, this might even drop the item directly into the player characters’ hands.
Bounties. Prolific thieves almost certainly have bounties for their capture, and even if the party cannot subdue them, there are usually smaller bounties for information leading to an arrest.
Favors. Returning items to past victims, preventing heists, and even just ending a spree of thefts can all ingratiate a band of roving sellswords with the community enough to receive some sort of preferential treatment, discounts, or laud.
Treasure and Equipment. Thieves steal a lot of stuff. They also frequently use the same kind of gear as adventurers, and they keep the best they can lay their sticky little fingers on. There could easily be chests of coin, magic items, and consumable adventuring supplies on the thieves or in whatever hidden place they use to keep their captives out of sight.
Information. Thieves tend to be streetwise. They hear lots of rumors and know secret paths in parts of the city most folk would avoid visiting. They also are the most likely people to know which noble suddenly came into a wealth of coins with strange demon faces minted on them or who has been smuggling huge quantities of pig blood for who knows what foul purposes. Whatever mysteries the party is pursuing in the city, they may coincidentally overhear a lead from a loose-lipped captor discussing business with another.
Enemy of my Enemy. All but the most iron-fisted underworld operations have rivals and enemies in the same business. Enterprising former captives could easily feed information on a gang’s hideout to a rival who would be willing to maintain a more amicable relationship with the adventurers in the future.