The Guns of Navarone

In 1943, the Axis powers plan an assault on the island of Keros â where 2,000 British soldiers are marooned â to bully neutral Turkey into joining them with a display of their military strength. Rescue by the Royal Navy is prevented by two massive radar-directed superguns on the nearby island of Navarone. When aerial bombing efforts fail, Allied Intelligence gathers a team of commandos to infiltrate Navarone and destroy the guns. Led by Major Roy Franklin (Anthony Quayle), the team is composed of Captain Keith Mallory (Gregory Peck), a renowned spy and mountaineer; Colonel Andrea Stavrou (Anthony Quinn) from the defeated Greek army; Franklin's best friend Corporal Miller (David Niven), an explosives expert and former chemistry teacher; Greco-American Spyros Pappadimos (James Darren), a native of Navarone; and "Butcher" Brown (Stanley Baker), an engineer and expert knife fighter.
Disguised as Greek fishermen on a decrepit fishing vessel, they sail across the Aegean Sea, where they successfully kill the crew of a German patrol boat coming aboard to inspect them. Later in the voyage Mallory confides to Franklin that Stavrou had sworn to kill him after the war, because Mallory was inadvertently responsible for the deaths of Stavrou's wife and children. The team shipwreck on Navarone's coast during a storm; Mallory leads the climb up the cliff, but Franklin badly injures his leg; the injury later becomes infected with gangrene. While taking shelter in the mountains, Mallory stops Franklin from committing suicide and then lies to him: that their mission has been cancelled and that a major naval attack will be mounted on the Navarone coast, away from the gun emplacements. They rendezvous with local resistance fighters: Spyros' sister Maria (Irene Papas) and her friend Anna (Gia Scala), who was once captured and tortured by the Germans before escaping; the trauma rendering her mute.
The mission is continually dogged by German soldiers, and the group is eventually captured in the town of Mandrakos while trying to find a doctor for Franklin. Stavrou pretends to grovel as they're being interrogated, which distracts the Germans enough for the group to overpower their captors. They escape in German uniforms, but leave Franklin behind to receive medical attention. In due course, Franklin is injected with scopolamine and gives up Mallory's misinformation, as Mallory had hoped: the fort is emptied in preparation for the supposed attack. Upon infiltrating Navarone, however, Miller discovers most of his explosives have been sabotaged and deduces that Anna is the culprit; she is forced to confess that she is not mute after all, and that the Germans recruited her as an informer in exchange for her release. Mallory reluctantly prepares to execute Anna as punishment, but Maria shoots her instead.
The team splits up: Mallory and Miller go for the guns; Stavrou and Spyros create distractions in town, assisted by brave local residents; Maria and Brown steal a boat for their escape. Spyros dies in a stand-off with a German officer, and Brown from knife wounds inflicted during the boat theft. Meanwhile, Mallory and Miller infiltrate the gun emplacement, but set off an alarm when they seal the doors behind them. Mallory and Miller proceed further towards the battery, where Miller then plants explosives on the guns and hides more below an ammunition hoist â with a trigger device set into the hoist's wheels. The Germans eventually gain entry into the gun cave, but Mallory and Miller make their escape by diving into the sea, reaching the stolen boat. A wounded Stavrou is helped aboard by Mallory, thus voiding the blood feud between them.
The Allied destroyers appear on schedule, but the Germans find the explosives planted on the guns and begin to fire on the passing Allied flotilla. The hoist eventually descends low enough to trigger the hidden explosives, which sets off the surrounding shells into destroying the guns and the fort. Mallory's team safely reaches the British convoy, but Stavrou decides to return to Navarone with Maria, with whom he has fallen in love. Mallory and Miller, returning home, observe the aftermath of their success from a destroyer.

Question: When Mallory and Miller seal the doors behind them, what do they set off?
Answer: An alarm.