Yeah that part always seemed significant to me. It indicates to me someone was watching the house or so intimate with the family that they knew their typical schedules. So creepy-can you imagine them living normal life the days before it happened except someone may have been watching them?
Now what would be nice to know is where CB had to be that morning. If he was always leaving at the same time everyday for a job or something then yes, someone could have been watching the house, BUT if he was usually home and going out that morning was not a usual thing then it had to be someone who was close to him or someone to whom he may have told he was going out early.
Oh and another thing, people in college do not always have class at the same time everyday. Wonder if this was the case with Holly? If so then this shows that someone had intimate knowledge of her scheduel.