The flares were shot with such incredible precision, if they stayed put they would have been encircled by them once they touched ground. At the time, they didn't notice this and continued walking towards the city.
It's walls were taller than those of the megacity of Texas, but the city itself was significantly smaller. Not just that, there were holes in the wall, too high to have been made by a roaming beast. The more they looked at the state of the wall, the more it looked like it suffered a recent attack.
After a couple minutes, new flares were shot their way, only this time the turrets mounted on the wall also shifted their position to directly face them. This time, the message didn't go over their heads and they remained still until the flares touched ground around them making a perfect circle.