In the future, when asked, he would never be able to explain how, exactly, he'd found Peter Pettigrew. But he found him, all the same. There was probably magic involved, for all that he didn't remember doing any. He must have done tracking spells, and anti-Apparition jinxes, and any number of things, to catch a wizard who didn't want to be found. Conspiracy theorists would later use it as so-called proof that the event had been preplanned, because tracking spells were extremely obscure and extremely complicated and had to be tuned to a person through extended effort. Remus Lupin, for his part, would later feel guilty about teaching Sirius to do it, years ago, with Peter as a practice target. But Sirius didn't know and never would; he only remembered the running.