In Mundungus Really A Thief?

Ok, so I was over at the MuggleNet forums and somebody mentioned in passing that Aberforth Dumbledore was present when Harry caught Mundungus “selling” Sirius’ stuff in Half-Blood Prince. I must have missed this the first time through, so I went back and read the passage. A few things about the passage struck me as quite odd, and I’m wondering if anybody agrees with me.

First there’s Aberforth. The beginning of the passage states: (bold emphasis mine)

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.

We know that the Hog’s Head barman is Aberforth. He’s obviously meeting Mundungus and when he sees Harry, he immediately takes his leave. What’s more, he draws his cloak around his neck as he does. To hide something? It was cold that day, but it appears that he is trying to hide his neck from Harry and co. Hmm, what could someone have on their neck that they’d try to hide? A locket perhaps? Ok, now skip ahead a few lines. Harry realizes that the stuff falling out of Mundungus’ case is from the Black mansion, and in a rage he pins him to the wall, choking him: (again bold italics mine)

“Harry, you mustn’t!” shrieked Hermione, as Mundungus started to turn blue.
There was a bang, and Harry felt his hands fly off Mundungus’s throat. Gasping and spluttering, Mundungus seized his fallen case, then — CRACK— he Disapparated.
Harry swore at the top of his voice, spinning on the spot to see where Mundungus had gone.
“COME BACK, YOU THIEVING — !”
“There’s no point, Harry.” Tonks had appeared out of nowhere, her mousy hair wet with sleet.
“Mundungus will probably be in London by now. There’s no point yelling.”
“He’s nicked Sirius’s stuff! Nicked it!”
“Yes, but still,” said Tonks, who seemed perfectly untroubled by this piece of information. “You should get out of the cold.”

Ok, first of all, why did Harry’s hands fly off Mundungus’ throat with a bang? It doesn’t appear that Mundungus has his wand out to do anything about Harry choking him, and it seems if he did he would have stopped Harry choking him before it got to the point where he’s turning blue. It must have been Tonks. Okay, but why? Why would she have allowed Mundungus to get away? I can see her stopping Harry from doing something drastic, but she could have at least gotten close enough to Mundungus to prevent his escape before she did it. And then her exchange with Harry is even more peculiar. She tells Harry that Mundungus will be in London by now and not to yell. How does she know where Mundungus is going? I realize the Order is in London, but there’s no particular reason for him to go there, so why does Tonks say that’s where he’s going when it would’ve made much more sense for her to just say “It’s too late, he’s gone”? She also seems to be pretty keen to get Harry to stop yelling and attracting attention. And then there’s her complete lack of concern that Mundungus is apparently stealing Black family heirlooms (which are her heirlooms too) and selling them to whoever. I realize that she is depressed about her situation with Lupin at this time, but shouldn’t she be just a little upset or angry? I mean, I’ve been depressed before and if anything it sets me on edge and makes my temper a little sharper.

So what, if anything, is going on here? Is it possible that Mundungus is on some mission for the Order? That he’s transferring Black artifacts to others for a reason? Maybe Aberforth is a spy for the Order? Maybe Dumbledore arranged for him to take certain items into safekeeping? But then, Dumbledore seemed to believe that the locket was in the cave. So if it was the locket on Aberforth’s neck, that would indicate that both Mundungus and Aberforth are doing things behind Dumbledore’s back. Could Aberforth be a Death Eater? Or is he just a greedy opportunist like Mundungus? As you can tell, I don’t really have a theory here, it’s just that this passage raises so many questions. I’d like to know what others think of it.