What do you think the guys thought was happening to Remus before they found out he was a werewolf?
Oh my gosh they were like eleven and twelve when they were wondering about it so they came up with some really, really stupid stuff. From a secret club, to a ministry spy, they came up with it. Peter half convinced them that he came back so weak because he was involved in ancient, Egyptian magic that drained a wizards energy but needed to be preformed by a chosen few every month in order to keep a deadly, secret force contained lest it be let free and consume the world and cited his dads background as a cursebreaker as reason for being privy to this topsecret information.
It took a few spooky, late night tellings of this story before James and Sirius finally got worried enough to research this terrible, deadly thing that their quiet roommate was potentially responsible for keeping at bay. Peter almost wet himself laughing because “I didn’t think you actually thought it was serious! We’ve just being making rubbish up right?“
Eventually it wore them down to the point of “You know what, maybe his sixth grandparent did die, who knows” and it was only when they’d pretty much given up coming up with ridiculous ideas that they noticed a pattern in the dates.