Other mistake: Near the beginning, Ana speeds away from her home down winding streets in the suburbs. In an aerial views of the houses, look closely at the row of homes at the bottom of the screen. None of the homes are shown with driveways or sidewalks. Essentially, there is no way getting to these homes. This is revealed in the commentary by the director who realized the mistake, but didn't care.
Other mistake: When Kenneth and Andy exchange via white board about Fort Pastor, you can see a guy chilling on the roof of a building nearby in the background just over Andy's head when he ask " so what's the bad news." He is not trying to get the attention of either Kenneth or Andy. (00:47:28)
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).