Plot flaws? [Spoilers]
The killer was enough of a lock-picking expert to get into the house without using any force, even though we saw a close-up of THREE locks on the front door -- that shot was doubtless in the movie to prove to us that after her kidnapping, Seyfried's character had got lots of security on the house.
So, why does the killer rent a van from an unreliable person -- a very risky thing to do -- with which to do the kidnapping? Why not just steal a van or car if you can pick locks? Why doe he rent a *locksmith's* van?
And second, why does he honk the horn twice when he has just finished committing the crime, alerting the neighbour on the opposite side of the street to his presence?