Vantage Point

U.S. President Henry Ashton (Hurt) attends a political summit in Salamanca, Spain, to promote an international treaty. Displayed with eight differing viewpoints, an assassination attempt on the president occurs, relayed in a time span of 23 minutes. Each time the events unfold from the beginning, a new vantage point is shown revealing additional details, which ultimately completes the story of what actually took place during the incident.[2]
From the first vantage point, GNN producer Rex Brooks (Weaver), directs various media personnel from a mobile television studio as the president arrives at the gathering. Mayor De Soto (Rodriguez) delivers a short speech and then introduces the president, who is shot twice as he greets the crowd from the podium. An explosion outside the plaza soon follows. Moments later, the podium itself is destroyed by a secondary explosion, killing and injuring numerous people. As the smoke clears, GNN reporter Angie Jones (Saldana) is seen lying dead in the rubble.[2]
The second vantage point follows Secret Service agents Thomas Barnes (Quaid) and Kent Taylor (Fox). Barnes notices a curtain fluttering in the window of a nearby building that was allegedly vacated. He also observes American tourist Howard Lewis (Whitaker) filming the audience. After the president is shot, Barnes tackles a man rushing to the podium named Enrique (Noriega). Taylor pursues a lead to a potential assassin. Following the second explosion, Barnes barges into the GNN production studio and asks to view their footage. He calls Taylor, who reports the direction of the suspected assassin's escape route. Barnes then views an image on one of the camera's live feeds that startles him and prompts him to run out.[2]
In the third vantage point, Enrique, a Spanish police officer assigned to protecting the mayor of Salamanca, sees his girlfriend Veronica (Zurer), being embraced by a stranger and overhears them speaking about meeting under an overpass. When he confronts her, Veronica assures Enrique of her love for him as he hands her a bag. When the president is shot, Enrique rushes onto the stage to protect the mayor, but is tackled by Barnes. While being detained, he witnesses Veronica toss the bag he gave her under the podium, causing the second explosion. Enrique escapes as the agents who previously had him in custody mount a chase while firing shots in his direction, failing to subdue him. Enrique confronts an unseen individual at the overpass and asks if he is surprised to see him still alive.[2]
The fourth vantage point revolves around Howard Lewis who is chatting with a man called Sam (Taghmaoui), while a little girl named Anna (Zapien), bumps into him and drops her ice cream. Later, Lewis notices Barnes looking at the curtain fluttering in the window of a nearby building, and captures the footage with his camcorder. Following the second explosion at the podium, Lewis chases Enrique and the pursuing Secret Service agents. At the overpass, Lewis views the pair of agents from afar shooting in the direction of Enrique as he greets an individual in a police uniform under the overpass. Seriously wounded, Enrique falls to the ground. Lewis sees Anna who had earlier become separated from her mother, trying to cross a busy intersection. An ambulance races down the road about to hit Anna, as Lewis runs out to save her.[2]
The fifth vantage point begins as President Ashton, having been informed of a credible assassination threat, has returned to his hotel room with his aides while his body double proceeds to the gathering in the plaza. The President talks with his personnel about the reason for the terrorists' plot, the retaliation by the U.S. to the plot, the return of Barnes to active duty, and giving the order for the retaliation to proceed. The first explosion occurs just outside the hotel. Seconds later, a masked assailant bursts into the president's room, shoots his advisers and then proceeds to abduct Ashton.[2]
By the sixth vantage point, terrorist Suarez, previously seen as Sam, shoots Ashton's body double using a remote-controlled automatic rifle placed in an adjacent window next to the one with the fluttering curtain that had drawn Barnes' attention earlier. The rifle is retrieved by Taylor, who Barnes sees leaving the scene wearing a Spanish policeman's uniform on one of the GNN live feeds, even though he tells Barnes that he's in pursuit of the assassin over the phone. Barnes realizes Taylor is actually part of the terror plot. The man Enrique saw embracing Veronica is revealed to be sharpshooter Javier (Ramirez), whose brother is being held hostage to ensure Javier's cooperation with the terrorists. The first explosion, at the hotel, is revealed to be a device detonated by a suicide bomber disguised as a bellhop who had previously handed Javier a hotel room key. Javier kills the guards and aides within the hotel, and kidnaps the president. Ashton is later placed in an ambulance with Suarez and Veronica disguised as medics. Javier joins Taylor in a police car to a planned rendezvous at the overpass. Barnes commandeers a car and chases Taylor and Javier. Barnes gets into a collision with a truck, allowing the duo to escape. At the overpass, Enrique, who did not die in the blast at the podium as intended, confronts Javier and Taylor. Enraged, Javier shoots Enrique, mistakenly believing he had knowledge of his kidnapped brother's whereabouts. Javier is then shot and killed by Taylor when he demands to be brought to his brother, who had been killed earlier by Suarez. Enrique dies of his wounds as Barnes reaches the scene on foot firing several rounds at Taylor, who attempts to flee. After crashing his car, a critically injured Taylor is dragged out by Barnes. He orders Taylor to reveal where the president has been taken, but Taylor dies. Meanwhile, Ashton regains consciousness in the ambulance and attacks Veronica, distracting her and Suarez just as Anna runs into their path. Suarez swerves causing the ambulance to flip over just as Lewis pulls Anna out of its way. Barnes runs to the ambulance where he sees Veronica lying dead. He shoots Suarez dead and rescues the president.[2]

Question: What does Barnes notice fluttering in the window of a nearby building that was allegedly vacated?
Answer: A curtain