Not criticizing.... just conversing.... While it may not seem the best way to abduct a person (don't like the sound of that) - it worked for whomever did this. To me it shows planning, and even experience - as if the perp had been watching Holly and knew this was the best way to grab her..
Hopefully, LE knows Holly's every-day routine and things she did recently and cab use that to figure out why the perp(s) took her this way.
It did apparently work, you're right on that account.
If Clint didn't see / hear anything happen, or he was not at home, one would think that even if Holly's vehicle was there, the family would not have suspected foul play immediately.
The mother would presumably return at 3-4pm and probably call to see if anyone picked up Holly on the way into school. She had a test that day and had been studying since 4am.
Perhaps the mother would think that some friends from a study group picked her up to go over the exam material. Or perhaps the mother would think that after the exam, depending on her schedule, she came home, some friends picked her up and they were enjoying a night of relaxing.
Either way, the perp would have had about a 7-8 hour head start if things went to plan. Not to mention that people would be skeptical of foul play as most abductions do not occur at a secluded house at 7am.
Clint and the blood in the garage were the two large problems. As soon as Holly bled on the ground, the situation changed. People would know something serious happened to Holly and that her life was in danger.
Clint being home and actually seeing the abduction take place was another huge stroke of luck in the effort to recover Holly. The police had the relative description of the suspect, a time, and a direct the abductor headed. All of those things are a rarity in so many missing people cases.