EWOT: The Rule of Efficient Pens

Lately in Professor Rizzo's class we discussed the rule of law and efficiency. It is basically when others leave their belongings unattended, we have some sort of notion to leave it alone. This is some sort of value law that states that we will not touch others property without permission. We also should not be rude to one another and try to put it down while you sit on the seat because that spot was somewhat to be reserved. But is that always the case? What about when you borrow a pen from your friend because you left it back in your dormitory. Most of the time you forget to give it back to the friend because you forget it's theirs and not yours. But who's to say you have to give it back to him? Is it just efficient not to give it back to him because he might have the potential to not want the pen back from you? If that's the case, then who's to say who's right and wrong? Is it then wrong to keep the pen from him or should you try to give it back to him because it was his possession to begin with? I am getting off topic from the rule of law and efficiency but I do believe these all apply to the metaphoric law we have hovering around us and the decisions we conduct in society today.