While in Hogsmeade with Ron and Hermione, Harry sees and follows Minister Fudge, Professors Flitwick, Hagrid and McGonagall and Madam Rosemerta. The trio end up over hearing the conversation between the group about Sirius Black. They explain that Sirius Black was the best friend to James Potter and that he was Harry’s godfather. They then explain that Dumbledore tried to hide them under a Fidelis charm which requires a secret keeper, and they chose Sirius Black for that. They only way that Voldemort could have gotten too them was if they were betrayed.
It was also revealed that when Black made a run for it, Peter Pettigrew tried to stop him, the resulting fight ended in the death of Pettigrew as well as 12 muggles. No body was found for Pettigrew except for a finger. More troubling was the fact that Dementors didn’t effect Sirius Black at all.