The Next Three Days

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Lara Brennan (Elizabeth Banks) is convicted of murdering her boss and is sentenced to life in prison. The evidence seems impossible to dispute: many colleagues saw her having a quarrel with the victim, their cars are parked right next to each other, she is seen leaving the crime scene seconds before the body is discovered, the murder weapon (a fire extinguisher) has her fingerprints on it and she has the victim's blood on the back of her overcoat. Following the failure of her appeal, her son Luke[6] ceases to acknowledge her during prison visits. One day, Lara attempts suicide, unwilling to spend the rest of her life in prison. Her husband John Brennan (Russell Crowe), a professor at a community college, becomes obsessed with the idea of breaking her out of jail.
John consults Damon Pennington (Liam Neeson), a former convict who escaped prison seven times. Damon gives John advice, along with a warning that the initial escape will be easy compared to evading the police after that. Following Damon's advice, John begins his preparation. He obtains a handgun and fake IDs, and studies the map of Pittsburgh for escape routes. To get money, he sells their house's furniture and personal belongings. John attempts to break Lara out from the prison in which she is held but abandons the plan when he is almost caught testing a self-made "bump key" on an elevator.
When John is told that Lara will be transferred in 72 hours to another prison facility, he is forced to come up with an emergency plan. Unable to get the money from his house in time, he considers robbing a bank, but hesitates at the last minute. Instead, John tails a local drug dealer to a drug lord, then robs him. Following clues left behind at the drug lord's house, the police track down John's car, get to his empty house and conclude that he is planning to break his wife out.
John tears down the big map filled with notes and photos of his escape plan, and scatter the pieces. He leaves Luke at his classmate's birthday party, then plants falsified blood work indicating that Lara is in a state of hyperglycaemia. Lara is transferred to a nearby hospital, where John convinces her to escape with him.
John and Lara leave the hospital, narrowly escaping the police and leave the city center. They then find out that Luke is at the zoo for the birthday party. John drives there to retrieve him, by which point the police have already established roadblocks on all interstate routes. Anticipating that police are looking for "a couple and a child", John improvises by picking up an elderly couple. They drive through the checkpoint without incident and proceed to Buffalo, New York, where John drops off the couple. The Brennans cross the border into Canada and head to an airport. Meanwhile, John's parents refuse to cooperate with the police. The police examine the escape plan fragments to figure out his destination, but are misled by the photos and delay the wrong flight.
A detective returns to the crime scene where Lara's boss was killed. He remembers Lara saying a button popped off as the mugger bumped into her, and deduces that it must have fallen in the storm drain. He searches the storm drain but is unable to find the button. It turns out the button was there, buried under grime, and the detective just missed it. While the police haven't the evidence to exonerate Lara Brennan, the audience does, because a flashback reveals how her boss died at the hands of a mugger, and how Lara came to have the victim's blood on her coat, and her fingerprints on the murder weapon. At the end of the film, the family arrives at a hotel in Caracas, Venezuela. As Lara lies down next to him, Luke kisses his mother and falls asleep. John takes a picture of their sleeping faces as the movie ends.

Question: What piece evidence is the detective searching for?
Answer: a button