Magic parchment that reveals the current location of anyone on Hogwarts grounds
‘Psst – Harry!’ He turned, halfway along the third-floor corridor, to see Fred and George peering out at him from behind a statue…
Harry Potter and the Prisoner of Azkaban
By J.K. Rowling
The point is, even if you’re wearing an Invisibility Cloak you show up on the Marauder’s Map.