Otto, I agree with you completely, except for the last part of the last sentence. I don't believe ANY realtor can access the lockbox, it has to be the agent who is handling it, or another agent with the same agency who is showing it as a favor. Real estate agents work on commission, so what if someone made an offer on the house while the substitute was showing it? Who gets credit for the sale?
IMO, though, I do agree that any house listed with a real estate agency would not be a good place to hide two girls or to molest or kill them, because some prospective buyer along with the agent could walk in at any time.
Someone mentioned several pages back about workmen or painters being able to access the code to get in... I believe the agent would have to meet them there and let them in. I would not trust my property to any agency who allowed their agents to go around handing out the keys or the code to the lockbox to just anybody. It would be limited access only.