i know that not everyone on my friends list is into HP, so i've cut this so as not to confuse the "grown-ups"... ;D
This theory is not an official canon-based theory, but it is a very logical theory if we assume two other popular, but unproven theories are true. Please take this as you will, since it is not an official theory. i was re-reading HBP last night, and this passage struck me as very fishy. i've tried to find a way to reason it out, and i think this theory makes a lot sense. Before the actualy theory, here are summaries of what you need to accept as true (or highly probable):
1. The barman of the Hog's Head is Dumbledore's brother Aberforth.
2. The locket found in Grimauld Place in OOTP is the true Horcrux.
Assuming that both of those theories are true, here is my theory:
Mundugus (not Kreature!) stole the locket after Sirius's death. He then sold it to Aberforth while in Hogsmeade. Which means that Aberforth is now in possesion of the Horcrux.
i know the obvious question is Why? Here's how i came to my conclusion:
We know that Mundugus was in Hogsmeade selling items he had stolen from Grimauld Place. In HBP (US pg. 245) Harry meets Mundugus in Hogsmeade. Mundugus is carrying a suitcase full of items he is selling (pawning). Harry recognizes a goblet as once belonging to Sirius. He accuses Mundugus of the theft and Mundugus flips out and disapparates. This means that it is possible that Mundugus also had the stolen locket in his case of items to sell.
But why Aberforth/the barman?
Before Harry confronts Mundugus he sees him from afar, standing with a "tall and thin" man who he recognizes as the barman. Here is the exact passage:
"...The exceptions were two men a little ahead of them, standing just outside the Three Broomsticks. One was very tall and thin, squinting through his rain-washed glasses Harry recognized the barman who worked in the other Hogsmeade pub, the Hog's Head. As Harry, Ron, and Hermione drew closer, the barman drew his cloak more tightly around his neck and walked away, leaving the shorter man to fumble with something in his arms..."
Based on this, we know that the barman was at least speaking with Mundugus. We can assume that they were bargaining. It is possible, then, that the barman bought something from Mundugus, something that had previously resided in Grimauld Place. To assume that he bought the locket is, i admit, a huge guess. But, there is one little hint which may imply that it was the locket:
"the barman drew his cloak more tightly around his neck..."
Not his body, his Neck. That is not much of a clue, but it does support this theory as a possibility. And there you have it. i think Aberforth/the Barman has the true Horcrux. i hope i've at least given you all something to think about!
Cheers! :D