In Prisoner of Azkaban, Draco Malfoy makes the follow taunts at Harry:
‘Thinking of trying to catch Black single-handed, Potter?’
‘Yeah, that’s right,’ said Harry offhandedly.
Malfoy’s thin mouth was curving in a mean smile.
‘Of course, if it was me,’ he said quietly, ‘I’d have done something before now. I wouldn’t be staying in school like a good boy, I’d be out there looking for him.’
‘What are you talking about, Malfoy?’ said Ron roughly.
‘Don’t you know, Potter?’ breathed Malfoy, his pale eyes narrowed.
Malfoy let out a low, sneering laugh.
‘Maybe you’d rather not risk your neck,’ he said. ‘Want to leave it to the Dementors, do you? But if it was me, I’d want revenge. I’d hunt him down myself.’
Prisoner of Azkaban, chapter seven (“The Boggart in the Wardrobe”), p. 96–97 (UK/Bloomsbury paperback)
As we find out later in the book, this is referencing Sirius’ responsibility for Harry’s parents’ death by betraying them as Secret Keeper of the Fidelius Charm protecting them.
Why does Draco do this, though? It seems rather strange, considering that:
The fact that Sirius was Lily and James’ Secret Keeper was not widely known (Fudge says when telling Madam Rosmerta about it that it was kept out of the papers and kept under wraps), so it’s not something Draco would be likely to have just come across—certainly not at the time, when Draco himself was only a year old, but also not now.
Draco’s father, Lucius Malfoy, who has his fingers in lots of Ministry pies and is frequently Draco’s source of confidential and insider information, was a Death Eater when Lily and James were killed, as far as we know among the Death Eaters closest to Voldemort. He, along with the other Death Eaters close to Voldemort, would surely know that Sirius Black was never a Death Eater nor a traitor, and they would almost certainly know that Peter Pettigrew was a Death Eater and was the one who betrayed Lily and James.1
If Lucius had ever told his son anything about this whole spiel, it would presumably have been the truth: that Peter had been a Death Eater (or spy) who had betrayed the Potters, divulging their Fidelius secret to Voldemort and bringing about their deaths; but that Sirius had been successfully framed for it. Otherwise, the most likely scenario would be that he just hadn’t told Draco about it at all (which we can probably rule out). There seems to be no logical reason for Lucius to give Draco confidential information that he knows to be false, as though it were truth.
So why does Draco deliberately lie and taunt Harry about going after Sirius, then? He is presumably one of the relatively few people who are aware that Sirius was never a Death Eater and has no reason whatsoever to be after Harry. The most he could possibly hope to accomplish would be for Harry to tear off seeking out Sirius. This would get Harry out of Hogwarts and out of Draco’s hair, but would also be one hell of a long shot.
1 That is, assuming Peter really was a Death Eater at that time. There is no mention of him having a Dark Mark until Goblet of Fire, at which point he certainly was a Death Eater, so it is possible he was only a spy during the First War, rather than a fully-fledged Death Eater. Even so, Sirius’ allegiance and Peter’s betrayal must surely have been familiar to Voldemort’s closest circle of Death Eaters.