Usually when you kill every enemy except for one, he will either shoot his gun straight up, or shoot a wall. If you wait about two minutes, about three or four more people will appear. When they shoot the wall or straight up into the sky, they are actually calling for backup. When another soldier far away hears a gun firing, they will run towards it to aid their friends. If you find a group of people and they all are killed except for one, and he begins to sprint away, it is imperative that you catch and eliminate him immediately before it is too late.