Ellie and a few other of the kids hear fighter jets fly over, it was strange because the jets were flying really low and without their lights on. The jets were flying late at night and in attack formations.
In chapter 4 Ellie says "Suddenly, the loud buzzing became a roar and like black bats screaming out of the sky, blotting out the stars, a V-shaped line of jets raced overhead. Their noise, their speed, their darkness frightened me. (Chapter 4)"