The team did run into some bad luck when the enemy soldiers barged into a room that had an angle on Athena from the door itself. Vreta did mark them, but it gave him less time to warn Athena than if they had approached from any other room. Still, Vreta responded right away in giving her cover. They committed onto taking out Athena, so Vreta was able to focus one target down to score a kill, and Athena was able to make it to cover, if only narrowly. 

Being that they were supersoldiers, Carthage and the rest of the Sacred Band had distinctive silhouettes, so Vreta knew right away when he had detected her. It took a moment to transition his aim, but he was able to see her the moment she was in view near the fire exit. He saw the launcher she wielded, and his implant could estimate its trajectory based on her current point of aim. She might have been able to take advantage of the opportunity, but, perhaps for reasons of pride, she inadvertently gave Vreta the time he needed to respond. The time she took to taunt Kjartan was only seconds, but on the battlefield, seconds may as well have been an eternity. Those seconds were enough for Vreta, while keeping low and out of sight, to retreat back a sufficient distance from the bus to keep his shield from breaking. Carthage had picked her target well, as Kjartan was not an immediate threat to her at range, but she may not have expected the trio of shots that Vreta sent through the fireball onto her position.