Go to the Entrance Hall. Go in the small custodian room to the left of the large 
exit doors. Use Wingardium Leviosa on 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. Use Wingardium Leviosa on the mop near the dungeon 
stairs and clean the odd tiles by the large staircase. You will get Discovery 
Points.