That's easy, I am not talking about "reasonable adults". I am talking about adults who, perhaps, come up with a scheme and stage an abduction, either for monetary gain or some sick way of getting attention. Most often, when someone tells a lie, the story they have come up with includes details that aren't needed, or details that are exaggerated. Bikes laying in a bike path, a purse on the other side of the fence, a voice mail that is discovered a week after a child goes missing COULD all be completely staged to appear as though an abduction happened. I don't think the girls left the bikes on that path, I think they rode them to another location, left them there, and went on a last minute trip with someone close to them. If they are camping somewhere, then that would ensure they would come across very few people, and they probably wouldn't have access to electronic or printed media. As for the purse, I would think that a 10 year old just has a purse to "have one", but it wouldn't be needed to carry ID or money necessarily, so it probably could be left behind without much thought, if it even was carried at all on that bike ride in the first place.
Like I said, this is solely a theory of mine, and in light of the last presser may not be completely out there.