jodiphour,
I think you are a little confused about what Rothbard is saying regarding just owners. Firstly, there is this except from the chapter I linked to:
Secondly, as I stated in a post earlier, if the just owner cannot be found, this is another way of stating that the property is unowned. That is, no one is making a claim of ownership. Since there is no claim of ownership, there is no owner. So the object in question may be passed on to a new just owner. Consider the dropping of a $20 bill. If you don’t see who dropped it, you have no idea who the just owner is. For that matter, the just owner doesn’t know where it is either. No one claims the $20. You become the new just owner.
If someone were to come along and claim that the $20 is his, do you believe him? Doesn’t he require proof? Are you just going to hand this $20 over to whoever claims it is his? Maybe you might, and maybe you might not. But there would at least be a claim by someone regarding the $20. He may or may not be the just owner of the $20, but he is making a claim to own the object in question.
In the case of not being able to identify the just owner, there is just no claim. As far as I’m aware, there needs to be a claim to ownership for there to be ownership. It is a necessary condition of ownership. Without it, there is no ownership.