This disused classroom is located in the Charms Corridor on the third floor. It is next to the statue of the humpbacked, one-eyed witch which covers the hidden passageway to Honeydukes in Hogsmeade. It is in this classroom that Fred and George give Harry the Marauder’s Map (PA10).
"He nodded towards an empty classroom to the left of the one-eyed statue. Harry followed Fred and George inside. George closed the door quietly and then turned, beaming, to look at Harry."