Percussive Pickpocket

The next thing she knew, Colm and her bracelet were gone.

In short, a thief pickpockets something off of someone, disguising the act as an accidental bump on the street. The theory goes that a brief, seemingly harmless touch presents all the opportunity a skilled thief needs in order to take what he wants off an unwitting target.

The marking quality of this type of encounter is that the thief usually tries to make as small a deal about bumping into the victim as possible, hurrying away before the mark realizes what happened. This is usually accomplished by a quick "Sorry," and brisk walk away from the protagonist.

Bumping into someone in fiction can indicate a wide range of events, from an early romance, to introducing a clumsy character to showing that the new character is simply prone to shoving people out of the way. What these events all have in common however, is that it is usually made apparent right away why the collision took place. There are times though, when someone will bump into the protagonist, and despite a suspicious amount of attention being given to the moment, the protagonist will play off the event as nothing, and little further attention is given to the collision even though it obviously happened for a reason. In nearly every such case, it is almost guaranteed that the colliding stranger introduced in this scene is a pickpocket at work.

There are a couple signals that indicate this trope:

The thief tends to have a disproportionate amount of screentime/attention dedicated to him/her for such a brief moment.
The victim tends to brush off the encounter and assumes that it happened for no reason. The same goes for any of his traveling companions.
The thief doesn't make a big deal about bumping into the protagonist. The thief doesn't give his name, doesn't try to engage in any extended conversation with the victim, and generally tries to get out away from his mark as quickly as possible. In general, the scene is played off to be as seemingly insignificant an event as possible.

To make it even easier, the thief tends to look shady or untrustworthy in general. If he is child, you can expect him to be a Street Urchin. If the stolen item was something important to the plot, you can be certain that the protagonist and the thief will meet again later sometime after discovering the loss of his missing item, having deciphered what exactly happened.

This can happen in Real Life, but pickpocketing in the real world is usually more complicated than how this trope is typically portrayed in fiction. Professional pickpockets tend to work in groups, and employ a variety of hybridized techniques to distract their victims that involve far more than a simple bump on the street.

While the term "percussion" is usually attributed to the family of musical instruments, as a word it simply refers to the act of one body striking another, hence the trope's name.

A Sub-Trope of Five-Finger Discount. Compare to Affectionate Pickpocket, where contact with the mark is achieved in a more forward manner.

Examples of Percussive Pickpocket include:

Anime and Manga

Gintama: Seita and Gintoki first meet when Seita pretends to accidentally bump into Gintoki and steals his wallet. Hilariously enough, not only does Seita find out that Gintoki wasn't carrying that much money but he also finds out that Gintoki had pick-pocketed him at the same time.
Rurouni Kenshin: Yahiko tries to pull this on Kenshin. It doesn't work.
Samurai Champloo, Episode 7: Fuu's money bag is stolen in this manner by a Street Urchin trying to fence off enough money to pay for his mother's illness.
Slayers: A little boy does this to Lina Inverse. She catches him and makes him her slave.

Film

Tintin: The pickpocket in the 2011 movie.
- Or, you know, the one in the comic: All the Thompson twins initially remember of him is that they bumped into him.
Gangs of New York: Amsterdam is pickpocketed by Jenny in this exact manner.
In the film Harry in Your Pocket, James Coburn plays Harry, the leader of a band of pickpockets. Part of their stealing technique involves bumping into people.
In the beginning of Payback, the Porter has just recovered from his wounds and is out on the streets with no money. He spots a man with a passing resemblance to him so he bumps into the man to steal the man's wallet. Porter not only takes the cash but also uses the man's ID to make a number of fraudulent credit card purchases.
The Sting. While Doyle Lonnegan is walking through the train to the poker game, Billie (Gondorff's girlfriend) bumps into him and steals his wallet. He doesn't notice until after he loses big at the poker game and tries to take it out to pay off his debt.

Literature

Oliver Twist: Happens to Fagin when he runs into a gang of young pickpockets.
Raiders of Gor: A girl is sentenced to slavery for being a pickpouch (Goreans don't have pockets) of this type. Bosk buys her and insists that she keep her skills sharp by allowing her to steal anything in camp, with the caveat that she must return whatever she steals within an hour or face serious consequences.

Live-Action TV

Babylon 5: Happens to Sheridan, where the thief steals the comlink.
Crusade: Happens to Max. Dureena recovers it, and tells Max that the pickpocket was clumsy.
An episode of CSI New York's Cold Opening has Mac catching a pickpocket (who manages to hide his stash before they grab him) just before running into the Victim of the Week. They later find a security camera video of said pickpocket bumping into their suspect and realise that he stole a camera with vital evidence on it.
1960s Batman episode "The Joker's Last Laugh". Batman discovers a tiny loudspeaker built into Commissioner Gordon's cufflink. He deduces that the Joker, who is a "master conjurer" (AKA stage magician), brushed into Gordon and switched cufflinks. Batman then reveals that in that single bump the Joker also managed to wrap several feet of an induction-receiving antenna around Gordon's waist and down his left trouser leg! Gordon then remembers that an oddly dressed person did bump into him on the subway that morning, confirming Batman's theory.

Tabletop Games

Starblazer Adventures, based on the 1970's-80's British science fiction comic book. The "Bump and Grab" stunt allows you to take advantage of a distraction (such as bumping into someone) to pickpocket them.
Chaosium's Thieves' World RPG (1981). The Game Master's Guide to Sanctuary had a plethora of random encounter tables. A couple of the entries had pickpockets bumping into the PCs in order to steal from them. Other encounters mentioned a NPC bumping into a PC so players wouldn't think that everyone bumping into them was a pickpocket.
Witch Bunter: The Invisible World. The Grab and Run talent allows the user to brush into a target and relieve them of their belongings.

Theatre

In the opening scene of Cyrano De Bergerac, a pickpocket includes this technique in his lesson as he teaches some young trainees the art of pickpocketing while at the theatre.

Video Games

Assassin's Creed 2: Ezio can do this to random people on the streets.
Breath of Fire II: Patty does this to you a couple of times.
Fire Emblem 8, the Sacred Stones: Colm bumps into Eirika during one cutscene and steals her Lunar Bracelet. Eirika doesn't quite realize what happened until after Colm has escaped.
Mass Effect 2: During the mission The Crime in Progress," a volus on the Citadel accuses a quarian of doing this to him and stealing his credit chit. The chit was actually forgotten by the volus at the last place he went shopping.
In Saints Row the Third you can purchase the ability to do this as an upgrade.

Web Comics

The Order of the Stick: A pair of children in Sandsedge attempt to do this to Durkon.