Revealing mistake: As Ana is driving away from her home, when we see her car driving from a shot high above, we see a car crashing in to a gas station making a huge explosion. However the trees a few feet away from the explosion don't move. The shockwave would have made them react at least a bit. (00:09:20)
Revealing mistake: When they use a gas cylinder and shoot it with a shotgun to wipe out a load of zombies, the resulting explosion as seen from above the trucks is way too big to have come from that small cylinder.
Revealing mistake: When Ana and Luis shower together at the beginning of the movie, Ana's hair is very long, almost down to her waist. This is obviously a body double as the real Ana's hair is much shorter.
Revealing mistake: When the very pregnant Luda washes blood off of herself, she leans over the sink. The actress' very flat tummy is visible underneath her red top.
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).