In Sorcerer's Stone we find Fred and George checking out a new passageway out of school:
"I tell you, we're going to win that Quidditch cup for sure this year," said Fred. "We haven't won since Charlie left, but this year's team is going to be brilliant. You must be good, Harry, Wood was almost skipping when he told us."
"Anyway, we've got to go, Lee Jordan reckons he's found a new secret passageway out of the school."
"Bet it's that one behind the statue of Gregory the Smarmy that we found in our first week. See you.
But shouldn't they know there's no new passageways out of school? They have the Marauders' Map!