"So [Filch] hauled us off to his office and started threatening us with the usual -- detention, disembowelment -- and we couldn't help noticing a drawer in one of his filing cabinets marked Confiscated and Highly Dangerous. Well, what would you've done? George caused a diversion by dropping another Dungbomb, I whipped the drawer open, and grabbed -- this."
-- Fred Weasley, about obtaining the Marauder's Map (PA10)
Filch has a drawer in his filing cabinet marked this. Naturally, the Weasley twins dove right in and discovered the Marauder’s Map (PA10).
Remus Lupin told Harry he also found a Boggart for Patronus practice lurking in the filing cabinet (PA12). It’s possible he was looking for the Map himself, since he was one of the authors.