If not Snape, then Dumbledore. Hell, they had Mad-eye! The most paranoid wizard in existence. He should have picked something up.

 

Yeah no doubt bout his l33tness, was just clarifying on Snape.

 

Madeye practices the 'do as I say, not as I do' type of Constant Vigilance. What with being outdone by a chap who'd been in Azkaban and the Imperious, and another who'd been a rat for a decade. Granted, he thought they were both dead, but one has to wonder why he didn't know there were two dead guys at the door before he opened it...

That being said, all the more proof that only in Rowling's math-befuddled mind was Peter a spy for an entire year, passing on secrets. I maintain it was much more 'at the moment'.