it's a possibility that it was taken by Regulus Black. it is possible but if it is him, his middle initial would have 2 be 'A'. it said that the note that Harry found in the fake Horcrux was signed 'R.A.B' and there could be hundred or thousands of witches and/or wizards with those initials. but in book 5, Sirius explained 2 Harry that his lil bro Regulus Black had a grudge against Voldy. if it was him he could've stashed it at #12 Grimauld Place b4 he was personally murdered by Voldy. and since Mundungus Fletcher had stolen a bunch of Sirius' stuff, he could've stolen the locket, but only if it was Regulus who got it and he had hidden it at #12. and therefore Mundungus could've sold it 2 anybody, so it could be anywhere. if that is tru then this might take a while.