So he knew at least one point of entry wasn't armed. Once inside the house, he would be able to hear the phone ring if he set off the alarm---most security companies call. He couldn't be sure that once the Ramseys went to bed, they didn't set the alarm, and that there wouldn't be motion detectors. 'Course he may not have even thought about the motion detectors.

If the security system was an issue to the perp, then that wouldn't that rule out someone that had intimate knowledge of the Ramseys day to day living habits? An insider that knew they didn't set the alarm?