Harry continued casting spells on the Marauder's Map.
Merely showing names was not enough.
When his father and his friends originally created the Marauder's Map, their abilities had not been sufficient; they had only managed to tap into the power of the oaths bound within Hogwarts. An instrument like the one Dumbledore made—capable of detecting intruders who had not sworn those oaths—was what a proper map truly required.
Chapter 348