How to Steal a Million

Charles Bonnet (Hugh Griffith) is a highly skilled art forger who pretends to be an art collector. His daughter Nicole (Audrey Hepburn) disapproves of his forgeries and is afraid that he may get caught. At the beginning of the film, he lends a renowned statue of Cellini's Venus to the KlÃ©ber-Lafayette Museum in Paris as the centerpiece of an important exhibition. However, the statue had been forged by Nicole's grandfather and would fail even a basic analysis as to its materials and age.
That night, while Charles is at the exhibition's opening event, Nicole hears a noise. She finds Simon Dermott (Peter O'Toole) holding a Van Gogh that her father made recently. Accusing him of burglary, she aims an antique gun at him and accidentally shoots him in the arm. To avoid an investigation around the fake masterpieces, she does not call the police, but instead cleans Simon's flesh wound and drives him to his hotel. He suddenly kisses her goodbye outside.
Soon after, Nicole has a dinner date with wealthy American tycoon, Davis Leland (Eli Wallach). She is suspicious of Davis, believing he knows her father is an art forger. He confesses to Nicole he is obsessed with owning the Cellini Venus, and arranged the dinner in order to convince her to sell it to him. Relieved that he doesn't suspect anything, she kisses him and tells him that the statue is not for sale.
The next day, an employee from the museum arrives at Charles's home and asks for Charles's signature on the one million dollar insurance policy for the sculpture. After signing it, the man reveals that Charles just agreed that the Venus will undergo a technical examination the following Friday. Desperate to save her father from being caught and sent to prison, Nicole meets with Simon, believing him to be a burglar, and asks him to steal the Venus for her. They both examine the museum's security measures, with Simon initially indicating he does not believe it is possible to steal the Venus.
On the night of the heist, Davis shows up again at Nicole and Charles' home before Nicole can leave to meet Simon. Davis is so in love with the Cellini and desperate to acquire it that he asks her to marry him. Not wanting to be late, she quickly takes the ring and leaves for the museum. Nicole and Simon hide in a utility closet until the museum closes. After making note of how often the guards check the museum, Simon sets off the security alarm surrounding the Cellini Venus using a boomerang he had bought at the park a few days before. The guards and police rush in and check the museum, but upon finding that nothing is missing, they soon leave and reset the alarm.
Simon reveals that he knows that Nicole wants the statue stolen because of the technical examination the next day, and suspected that the statue was a fake because it resembled Nicole (it was modeled after her grandmother.) He only helped Nicole because of his feelings for her, and they kiss. Simon then sets off the alarms again. After realizing that nothing is missing, the frustrated guards decide the security system has malfunctioned. Because of telephone complaints from high ranking politicians in the area about the alarm, they turn it completely off. Simon then steals the statue, and Nicole hides it in the cleaning woman's bucket and they escape out the basement in the confusion after it is discovered missing.
The next morning, after the news of the robbery has spread, Davis quickly looks for a lead on the missing statue, desperate to get to it at any cost. He meets Simon, who says he will give the Venus to him, but that he can never mention the statue to anyone, or see Nicole again. After Davis leaves, Nicole joins Simon at his table to celebrate the robbery. Simon finally reveals to Nicole (who still believes him to be a professional burglar) that he is in fact a college-educated art expert and investigator hired by the world's largest galleries to strengthen security and uncover forgeries. The Cellini Venus was, in fact, his first heist.
Later, at the steps of a private plane, Simon passes Davis the Venus. When he opens the box, Davis also finds Nicole's engagement ring.
Simon assures Charles that the fake Venus is safely out of the country. Charles is so relieved at the news that he's only momentarily disappointed when Simon tells him that the purchase price was, and will remain, zero dollars. Simon and Nicole extract a promise from Charles that he will stop selling forged paintings.
Nicole and Simon get married. As they leave the mansion, however, a South American collector who had enthusiastically admired Charles' new Van Gogh at the start of the film pulls up in a car and is warmly welcomed by the old forger.

Question: What set off the alarm in the museum?
Answer: Boomerang