Continuity mistake: In the beginning when Dr. Dandewar hangs up the phone, there is an orderly pushing the cleaning cart in the distance behind Ana. When it cuts to the close-up, the orderly disappears with the cart. (00:00:45)
Continuity mistake: In the beginning when Ana is standing beside Dr. Dandewar who is speaking on the phone we see her looking away from him. When it cuts to him hanging up the phone she looks at him and he has moved a feet away from the wall. (00:00:50)
Continuity mistake: When Dr. Dandewar has hung up the phone we see him holding the X-ray in his hand. When it cuts he is holding it with both hands. (00:00:55)
Continuity mistake: When Dr. Dandewar hangs up the phone you can see the phone chord gets twirled up. When it cuts the chord is straight. (00:00:55)
Continuity mistake: When Ana is done talking with Dr. Dandewar she gets to Cora's desk and places her files on the desk. When it cuts the brown file on the desk has moved. (00:01:15)
Continuity mistake: When Ana hands over the file to Cora, Cora grabs the file on the side. When it cuts she grabs it on the top of the file. (00:01:20)
Continuity mistake: When Ana is talking to Cora at the beginning, Ana has her elbow on the counter and her hand behind her neck. Her hand changes positions from shot to shot. (00:01:20)
Continuity mistake: When Ana leaves the hospital she approaches an ambulance. As the walkie-talkie scrambles she puts her arm on her hip. When it cuts her arm is down. (00:02:05)
Continuity mistake: The E.M.T.'s legs in back of ambulance when first shown are hanging over the red line on bumper. When they cut to farther shot, The legs are now pulled up more. (00:02:05)
Continuity mistake: When Ana is driving home we see an above shot of her car as she approaches Vivian. When it cuts to Ana stopping right next to Vivian you can see there a far more cars in the background than it was in the previous shots. (00:02:35)
Continuity mistake: When Vivian bites Luis Ana grabs Vivian, and some of Vivian's hair hangs over Ana's hand. When it cuts to Ana dragging Vivian off Luis the hair is under Ana's hand. (00:05:20)
Continuity mistake: At the beginning of the movie, when Ana is escaping from her husband, she crashes into the bath and in the process pulls down the shower curtain and rail. The shot from above shows the curtain rail landing on top of her, but when the shot immediately switches to show to show a close up of her sitting in the bath, the rail appears to be propped up just behind her right shoulder. (00:06:30)
Continuity mistake: When Ana and Kenneth are walking through the tunnel, as Andre fires at them, Kenneth is standing in the sunlight shining in to the tunnel. When it cuts to Kenneth and Ana bending down the sun isn't shining inside the tunnel anymore. (00:13:55)
Continuity mistake: When Andre is walking alone and gets to one of the mall's entrances he wipes his forehead. When it cuts he is wiping his eye. (00:20:10)
Continuity mistake: When they are backing up the truck, it hits the fence on the left side bending almost the whole fence. When it cuts the fence is suddenly intact. (00:41:05)
Continuity mistake: After Andy shoots the zombie that looks like Jay Leno, there are shots of the characters looking through binoculars. Tucker's and Steve's binoculars still have the lens caps on. (00:58:05)
Continuity mistake: When Michael breaks the safety glass for the axe, some of the glass remains intact. When it cuts the glass is gone. (01:01:25)
Continuity mistake: When Norma places the case on the table, Ana says she shall go check on Luda while placing a box horizontally in the case. When it cuts the box is lying vertically. (01:08:00)
Continuity mistake: After Norma has been killed and Ana says someone should say something, Kenneth moves forth while saying "There is nothing to say," and Nicole comes out from behind the counter and walks a few feet. When it cuts she is walking out from behind the counter again. (01:12:50)
Continuity mistake: When they are going to rescue Nicole from Andy's store, Michael shoots a zombie in the head and blood splatters on the side of the truck. The amount of blood on the side differs between the shots. (01:24:25)
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).