Глупый вопрос.
Почему Снейп не знал, кто из Мародёров был предателем?
В Хижине Сириус говорит, что наслушался в Азкабане проклятий в адрес Питера (the death eaters thought that double-crosser double-crossed them), что Питер все 12 лет прятался именно от них, и что упаси боже кто-нибудь из оставшихся на свободе проведает о том, что Питер жив.
UPD Добавляю цитату (по-английски; перевода у меня, к сожалению, нет)
"You haven't been hiding from me for twelve years," said Black. "You've been hiding from Voldemort's old supporters. I heard things in Azkaban, Peter... They all think you're dead, or you'd have to answer to them.... I've heard them screaming all sorts of things in their sleep. Sounds like they think the double-crosser double-crossed them. Voldemort went to the Potters' on your information... and Voldemort met his downfall there. And not all Voldemort's supporters ended up in Azkaban, did they? There are still plenty out here, biding their time, pretending they've seen the error of their ways.
If they ever got wind that you were still alive, Peter --"