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.
Captain Defenestrator
4th Jun 2008
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