Aberforth horcrux theory
OK, just to clear it with you, Aberforth (Dumbledore's brother) is the barman in the Hog's head. JK said so.
RAB is Regulus Black. He was going to destroy the locket but he got killed, so he left it in Grimmauld place. When the Order moved in, no one thought it was important. In the Half blood prince, we see Muundungus and the Hog's head barman (Aberforth) outside the 3 broomsticks. Mundungus is selling Sirius' stuff. As Harry and co. approach, Aberforth pulls his cloak tight around his neck and walks away. So could Mundungus have sold Aberforth the locket?
Abe obviously knew that it was a horcrux, as he tried to hide it from Harry. So why didn't he tell Albus? I reckon that he did, but Dumbledore did not know which horcrux was in the cave when he found it.