I believe it could have been someone she did not know. But I think they knew her...IOW, I think they knew enough about her comings and goings to know when was the best time to strike.
Or, it could have been a neighbor, someone Sierra knew but not very well. Someone who looks normal, acts normal. No one would have reason to suspect him. We've all heard after a perp is arrested for a horrible crime..."He was so pleasant and helpful, it's just not something we ever thought he would be capable of."
I can easily see a male neighbor just happening to be outside at the time Sierra left the house...maybe saying good morning to her and smiling, getting her into even a very brief conversation could delay her long enough to think she will miss her bus, and then he does the neighborly thing by offering to drop her off at school.
She could be in his house right now. In an attic, a closet, a basement, or crawlspace. Probably a place that has already been searched and thus cleared for the perp to use however he sees fit.
Maybe they offered to Drop her off because it was raining? If a neighbor did take her where would she have been when they searched his or her house?