context: Paul Kersey (Charles Bronson) and his wife Joanna (Hope Lange) vacation in Hawaii. They return to New York City, where Paul works as an architect.Joanna and their daughter Carol Anne (Kathleen Tolan) shop for groceries at D'Agostino's. Three hooligans (one played by Jeff Goldblum) are wreaking havoc in the supermarket. They catch Joanna's address after she asks that her groceries be delivered. They follow her to the apartment, burst in and trash the apartment. They search for money but find only $7. The hooligans then rape Carol and savagely beat Joanna, escaping scot-free.Paul's son-in-law Jack Toby (Steven Keats) calls to tell him only that Joanna and Carol are in the hospital. After waiting impatiently, Paul is told by a doctor that his daughter is okay and that she was sedated and put to bed, but his wife has died. Devastated, he is told by police that the likelihood of catching the criminals is small since his wife is dead and his daughter, the only witness, traumatized from her ordeal of being raped, cannot speak. That night, a frustrated Paul goes out for a walk and quickly encounters a mugger who attempts to rob him with a gun. Carrying a sock full of quarters, Paul swings it at the would-be-mugger and flees, narrowly escaping from being shot.The next day, Paul's boss gives him an extended business vacation to Tucson, Arizona to meet a client, Ames Jainchill (Stuart Margolin), who shows him the ropes. Paul witnesses a mock gunfight at Old Tucson, a reconstructed Western frontier town used as a movie set. At a gun club, Ames is impressed when Paul shoots near bulls-eye accuracy. He reveals that he was a "CO" (conscientious objector) during the Korean War who served his country as a combat medic. Ames drops him at the airport, slipping a little going-away present into Paul's bag.Back in New York, Paul visits Jack who tells him that his daughter is catatonic and will likely never recover. At home, Paul opens his suitcase and the gift box that Ames left him to find a nickel-plated .32 Colt Police Positive revolver. He pockets the gun and takes a stroll that night. While walking alone in Riverside Park, Paul almost immediately encounters a mugger, a doped-up ex-convict named Thomas Leroy Marston who attempts to rob him at gunpoint with a .38 Smith & Wesson Model 36 revolver. Paul shoots him with his own revolver, killing him.Shocked that he just killed a human being, Paul runs home and throws up. But his vigilantism continues the following night, when he guns down three black men (one with an afro is played by an uncredited Denzel Washington) who are robbing a defenseless old man in a vacant alley.A few nights later, two muggers see Paul on a subway, holding a bag of groceries. They attempt to rob him at knife-point but Paul shoots them both with the revolver. Paul again gets away.Another night later, Paul is sitting in a sleazy 8th Avenue coffee shop surrounded by prostitutes and assorted street people. He pays his bill to the cashier purposely revealing a wallet full of cash. He leaves followed by two black thugs, named George and Joseph, who have taken the bait. Yet again a robbery attempt is made in a subway tunnel. Paul shoots one but the other manages to stab him in his shoulder before he gets shot as well. As a wounded Paul stumbles off, the one who stabbed him gets away mortally wounded, dying at a hospital.Police Lt. Frank Ochoa (Vincent Gardenia) investigates the vigilante killings. His department narrows a list to men who have had a family member recently killed by muggers and who are war veterans. The public, meanwhile, is happy that somebody is doing something about crime.Lt. Ochoa soon suspects Paul. He is about to make an arrest when the District Attorney (Fred J. Scollay) intervenes and tells Ochoa to "let him loose" in another city instead. Ochoa doesn't like the idea, but relents.In the meantime, Paul has his daughter committed to a mental hospital in upstate New Hampshire where she will remain catatonic, possibly for life. After returning to New York, Paul hides his .32 gun in a hidden drawer in his office while adjusting (and possibly enjoying) committing murder against muggers to "clean up the city". When Paul invites Jack over for dinner, Jack cannot understand Paul's sudden and happy mood.(Note: The three muggers who murdered Paul's wife and assaulted his daughter are never seen again in the movie, so their fate is left ambiguous. Since Paul was not present during their attack, it is implied that Paul only targets random muggers to vent his frustration at not being able to find the ones who assaulted his family.)One evening, Lt. Ochoa anonymously phones Paul at his office to tip him off that the police are watching him, hoping to provoke a reaction or slip up from Paul. When Paul is briefly detained by the police after leaving his office, they do not find any gun on him and allow him to continue on his way.Aware that the police are starting to catch on to him, Paul gives them the slip by sneaking out of his apartment building through a backdoor service entrance. He goes to his office to retrieve his gun and then goes out looking for more would-be muggers where he is confronted by three street gang members in Central Park who try to mug him. Paul shoots two of them before he is wounded in his right leg by a third mugger who is armed with a .45 caliber M1911A1 pistol. Despite his wound, Paul chases the third mugger through the park, the streets, and to an industrial warehouse area where the third mugger gets away as Paul attempts to take aim at him, only to fall to the ground.Hospitalized, Paul is confronted by Ochoa after a sympathetic policeman finds Paul's revolver near the latest crime scene and gives it to the detective. Ochoa makes Paul promise to leave New York, permanently in exchange to avoid prosecution for the vigilante killings. Paul replies, "By sundown?"In the final scene, set one week later. Paul arrives in Chicago Union Station by train. Being greeted by a company representative, he notices a group of hoodlums harassing a woman. He excuses himself and helps the woman. The hoodlums make obscene gestures, but Kersey points his right hand like a gun and smiles... suggesting that his vigilantism will resume sooner or later.
question: Who discovers Paul's gun?
answer: a sympathetic policeman