Factual error: When the survivors are first in the mall, you can see a store called Roots with a Canadian beaver logo. While the movie was filmed in Canada, it is supposed to take place in the USA where Roots has a different logo. (00:17:05)
Factual error: When they are eating, there's a box of chicken labelled PFK (Poulet Frit a la Kentucky), the French equivalent of KFC. This film takes place in the USA, but was filmed in Canada where most products are labelled in English and French.
Answer: Since it's not addressed in the script or the film, any answer would be speculation. One could be that Luis is just a generally laid back person, and the appearance of a young child he knows is not enough to ruffle his feathers, even if she isn't supposed to be there. He also just woke up, so he could be a bit groggy and not fully processing the situation as quickly as he might normally. More likely, it's simply the way the scene was filmed; the director and actor didn't think it important to show Luis "bothered" by the girl's presence (prior to noticing the blood, of course).