Does it really need to be said, I do not own the books.
Why Remus think's it was Sirius:
"Peter, what are you getting at?" He asked cautiously. Having known him for many long years, Remus could tell when something was bothering any of his friends. Peter had been acting odd for a few months now, always avoiding the rest of them, hardly ever coming around to see Harry.
Lily said he was simply down, and still coping with the recent death of his mother, but the boys couldn't help but wonder if something else was getting to him. Now Peter had shown up at his door, asking if he knew where Sirius was, and swearing if Peter told him something he couldn't go telling Sirius.
Peter's eyes darted nervously to the door, then back to Remus, before he seemed to chicken out and blurt, "Nothing. I'm just being, ah you know. I'll be around Remus-" and before he had even finished speaking he was walking back out the door.
Remus tried to follow him, made to reach out and physically stop his friend from leaving, but with a loud crack, his old school friend had disappeared.
Remus wouldn't see him for twelve years.
Short and vague, but since Sirius had never told Remus that they had switched secret keepers, and eyewitnesses claimed Sirius had killed Peter, plus this little instance...well you can see how Remus might think the worst.