"Now, I don't suppose you'll be wanting some sort of reward for help-"
Lewis never finished checking things off. Luckily, he really had all the time in the world. In fact, with the new string of visitors he was about to receive, he would have had to redone the math anyways.
When he heard the THUNK he put his hand on his gun. He had initially assumed it was Mackinzie dropping something or something like that. It didn't take long to realize the sound had come from outside the fence. He unholsters his weapon and follows Mackenzie out. He says "I think you're just going to scare him off, Mackenzie." Beat. "Maybe it was just an animal." Then again, many animals were just as dangerous as zombies. In retrospective, perhaps cloning entelodonts and Unenlagia during the zombie apocalypse was a bad idea.