So here we are thinking about book VII : R.A.B

Hoooohooo!! The mysterious R.A.B who stole (and destroyed?) the locket horcrux that Harry and Dumbledore got is Regulus Black, Sirius's younger brother, ex death-eater from Slytherin and once taught by Slughorn, who was murdered a few days after leaving the Death Eaters. So what happened to the horcrux? Well, at the beginning of book V, when all are spring-cleaning Grimmauld Place, they come across loads of junk including a heavy locket that none could open. It is suggested in the book that they chuck it, and everything else, out. But it is quite plausible that Kreacher dragged it of somewhere. The question is whether a bit of Voldermort's soul is still in there.