Go to the Entrance Hall. Go in the small custodian room to the left of the large exit doors. Wingardium Leviosa the mop and hover it over each of the four odd tiles by the golden statue. It will clean them. You will get Discovery Points, and Hermione will talk, giving you a 100% on the Marauder's Map. Then, go to the Viaduct Entrance. Wingardium Leviosa the mop near the dungeon stairs and clean the odd tiles by the large staircase. You will get Discovery Points.