ok, Obiwan never owned any droids, R2 never had his mind erased, but it was ok, cause R2 only does his job without question, he doesn't yap all the time like 3P0, so it was safe to not erase R2's memory.
R2 claiming to be the property of Obiwan kenobi was only said because R2 didn't want Luke thinking that he owns him, and it makes it easier to find Obiwan because he needs to give the messege.
I find it funny that R2 didn't reconize the place where Owen and Beru lived on tattoine. In EP II, R2 was around. in EP IV, R2 didn't reconize Owen and Beru,or the land. maybe he did but just didn't say anything because hes got an important mission.