>saw a check stub or some kind of payment reciept for the bonus in the house.
>Even though that seems unlikely.

Why 'unlikely'?

IF the intruder saw the whole family troupe out to the Whites and knew he would have several hours to 'kill time' why not go rummaging about and see if he could find something interesting. There is not much question that a variety of personal papers where in the house in both the den area and the basement area.

If he had time to sit down and write this humongous ransom note he sure had time to wander about and pry into papers.

I don't think he would have considered 'snooping' to be unacceptable behavior if he considered strangling a six year old girl to be acceptable behavior.