That would be an amazing coincidence. But then, coincidences do happen. From the few pictures I've seen, it appears the house is at the end of a cul-de-sac. Seems that would make it highly unlikely that this was totally random. Might be more likely a worker on an adjacent house, or friend/guest of a neighbor. Strange that investigators think they left on foot rather than getting into a car out front, perhaps he parked on the street behind? My guess is that she was targeted.
Not trying to be a pollyanna, but I don't think we can yet rule out that she left willingly. Perhaps she arranged a staged act with a new boyfriend and didn't let her sister in on it. Teens can be ruthless and creative. Was she angry at her parents?
But still a stalker(either cyber or someone who took notice of her) seems the most likely theory. The only way I could picture it being random would be if someone saw her, say at the mall, followed her home, parked and observed movements, then came back later on a nearby street(assuming there was no backyard fence), and broke into the house.