Harry Potter & the Failure of the Marauder's Map
Every Harry Potter fan knows and loves the Marauders because they stand for friendship, loyalty, and (above all) mischief. The four pranksters (Moony, Wormtail, Padfoot, and Prongs) are notorious for sparking fan theories, forwarding the Harry Potter narrative, and creating the Marauder's Map.
The Marauder's Map is a map of Hogwarts School of Witchcraft and Wizardry. It is a magic map that shows the location and movements of anyone in the school. Harry Potter receives this gift from Fred and George Weasley when the lack of parental permission prevents him from visiting Hogsmeade with the rest of his class. The Weasley twins let Harry use the map because it shows the secret passages inside Hogwarts which Harry could use to sneak off campus. They tell Harry that they've had the map for years and that they stole it from Filch's desk.
Let's take a quick look at some of the map's failures.
- Fred & George had possession of the Marauder's Map during Harry's first year at Hogwarts. By using the map to sneak around, the twins would have seen Voldemort walking around with Professor Quirrell. Yet they alerted no one.
- The Marauder's Map would have detected Barty Crouch Jr. walking around the school while masquerading as Mad Eye Moody. It also would have shown that the real Mad Eye Moony locked up somewhere because he would always show up in the same location on the map.
- Fred & George should have noticed that their younger brother, Ron Weasley, had been sleeping with Peter Pettigrew for years.
- During Harry's second year at school, the year involving the Chamber of Secrets, the basilisk should have shown up on the map. The physical embodiment of Tom Riddle should have shown up as well.
J.K. Rowling and the folks at Quora have debunked some of these failures. Rowling has claimed that Voldemort didn't show up on the map when he was possessing Professor Quirrell because Quirrell was a "temporary horcrux" and the map does not detect those. If the map had shown Voldemort walking around with Quirrell it would need to show Voldemort's name next to Harry as well. I can understand that.
They have also brought up the idea that the Marauder's Map only shows areas that the Marauder's themselves knew about. This is the reason that the Room of Requirement and Chamber of Secrets don't appear on the map. However, the Basilisk still should have appeared because it lurked around the halls and petrified both students and animals. It might have shown up as "the monster" rather than "the basilisk" because people didn't know it existed- but it should have shown up.
These two excuses... ahem... explanations still do not address the problems of Peter Pettigrew, Barty Crouch Jr., or Mad Eye Moody.