Corrected entry: When they get to Debra's home, they open the door and have to disarm the alarm. She punches in the wrong number, and the alarm goes off. However, they find the family (now zombies) inside the house. The alarm should have gone off by the family's movements before the group got there.
Corrected entry: In the scene where Riley is in the kitchen with Deb and Tony, Riley says in reference to Cook "I wonder if she ate the rabbit". Later in the pool a male figure in a chef's outfit, presumably Cook, can be seen. This is even clearer in the DVD special effects featurette.
Correction: Riley refers to the staff as "Brannigan, Brenda, Cook." Cook is someone's last name not a reference to their occupation.
Correction: Home security systems often have settings for "Home" and "Away." The Home setting leaves the alarms on the doors and the windows operational in the event of a break-in, but disables the internal motion detectors so the alarm doesn't go off every time residents in the house move around. Since Debra's family made it home before everyone turned, they probably switched it to the Home setting when they arrived.
Captain Defenestrator