Didn't Fred and George ever wonder why this bloke named Pettigrew was always shown on the map in Ron's dorm?
My theory about the map has always been that you need to be looking for certain people, or at least have them in mind, when you are using the map in order for them to show up. Otherwise, everyone in the castle would show up every time Harry used the map, and that's not the case. He looks at the map and sees Snape, Filch, Mrs. Norris, Dumbledore, etc., but not every single student in the school. He sees Moody in Snape's office because he's thinking about Snape and wondering where he is, and so forth. Since everyone thought Pettigrew was dead, there was no reason to ever see him on the map. That's in the book, of course; the movie treated it differently, as you know.
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My theory about the map has always been that you need to be looking for certain people, or at least have them in mind, when you are using the map in order for them to show up. Otherwise, everyone in the castle would show up every time Harry used the map, and that's not the case. He looks at the map and sees Snape, Filch, Mrs. Norris, Dumbledore, etc., but not every single student in the school. He sees Moody in Snape's office because he's thinking about Snape and wondering where he is, and so forth. Since everyone thought Pettigrew was dead, there was no reason to ever see him on the map. That's in the book, of course; the movie treated it differently, as you know.