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In Order of the Phoenix, Snape eggs on Sirius at Grimmauld Place:

‘Tell me, how is Lucius Malfoy these days? I expect he’s delighted his lapdog’s working at Hogwarts, isn’t he?’ [Sirius]

‘Speaking of dogs,’ said Snape softly, ‘did you know that Lucius Malfoy recognised you last time you risked a little jaunt outside? Clever idea, Black, getting yourself seen on a safe station platform ... gave you a cast-iron excuse not to leave your hidey-hole in future, didn’t it?’
Order of the Phoenix - Chapter twenty-four, Occlumency - Page 460 - Bloomsbury

How did Lucius Malfoy know that Sirius was a dog Animagus? How did he know it was Sirius in the train station?

Some facts to consider: According to the Harry Potter Lexicon's timeline, the characters who are considered to be of the Marauders era (born 1959) were only five years younger than Lucius Malfoy (born 1954), so Sirius and Snape, and the other Marauders-era students, and Lucius Malfoy, would have been at Hogwarts together for two years. Prisoner of Azkaban demonstrates it took Sirius, James, and Peter four to five years to master the Animagus transformation, which would have been at least two years after Lucius Malfoy would have left Hogwarts (Although we don't know when Lucius became a school governor.).

A canon-based answer would be great; barring one, a speculative answer in the spirit of canon is welcome.

  • 15
    Their was probably a rat *wink – Himarm Dec 20 '15 at 22:01
  • 2
    @Himarm -- Indeed there was! :D – Slytherincess Dec 20 '15 at 22:24
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    For me the bigger question is why wasn't Sirius's Animagus form more widely known? With James, Lupin, and Sirius himself in the Order and Pettigrew among the Death Eaters, I'd expect everyone on both sides to know about it, yet even Dumbledore appeared to be in the dark until PoA. Hmm ... I could probably turn this into a decent question ... – Rand al'Thor Dec 20 '15 at 23:26
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Peter Petigrew told Voldemort who presumably told Lucius. Sirius speculates this much in Order of the Phoenix.

“Because the Ministry of Magic’s still after me, and Voldemort will know all about me being an Animagus by now, Wormtail will have told him, so my big disguise is useless. There’s not much I can do for the Order of the Phoenix…or so Dumbledore feels.”

-- Sirius Black speaking to Harry (Order of the Phoenix, Chapter 5)

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