[Harry Potter] How Pettigrew wasn't noticed by Fred and George in the Marauder's Map

So, for starters, we know that Peter Pettigrew, living as the rat Scabbers, was first taken to Hogwarts as Percy Weasley's pet. That was four years before the events of the first book. We later find out Fred and George Weasley – who started attending Hogwarts when Percy was about to start his third year – stole the Marauder’s Map from Filch’s office in their very first year. So when wondering: 'why didn't they see Pettigrew sharing a bed with Ron?', we must also wonder why didn't they see anything when Percy had him as a pet. Here's my take...

Pettigrew/Scabbers, being one of the creators of the map and free to roam around the school and/or other dormitories, could likely become aware the twins got into possession of the map. He couldn’t risk being spotted. So he started timing his appearances in the Gryffindor Common Room [or dormitory] when the coast was clear to him - when the twins were entertained with something else or had no use for the map. It was so perfectly timed that Scabbers’ appearances in the first two books gain another significance when we see them in this light:

First book: Scabbers is only mentioned in the Hogwarts Express chapter, and shortly after Ron and Harry are sorted into Gryffindor, in their first night at the school (‘Great food, isn’t it?’ Ron muttered to Harry through the hangings. ‘Get off, Scabbers! He’s chewing my sheets.’) After that, he’s mentioned twice: When Harry was about to play his first Quidditch match, referring to the sheets he chewed on months before (‘As a surprise for Harry, they had painted a large banner on one of the sheets Scabbers had ruined’) And then, being ‘physically’ present, in the winter break chapter, after Harry meets Dumbledore when searching for the Mirror of Erised (‘But then, he thought, as he shoved Scabbers off his pillow, it had been quite a personal question’)

Second book: Scabbers is only mentioned when he’s at the Burrow and being taken to Hogwarts in the flying car, until he appears in the chapter that also covers the Christmas break (‘‘Positive,’ said Hermione, shifting Scabbers the rat so that she could sit down on the end of his four-poster.’)

Overall: He’s only seen and described to be physically present with Harry and Ron in environments that wouldn’t be covered by the Map (the Hogwarts Express, the Burrow, the flying car). After getting to Hogwarts, he’s only mentioned to be in the Gryffindor’s common room or the boy’s dormitory during the Christmas breaks – and it should be noted that Fred and George also spent those breaks in Hogwarts, but they wouldn’t need to use the Marauder’s Map since the school was practically empty.

In the third book: Scabbers only starts hiding out in the boy’s dormitory after Sirius escapes. By then, Fred and George are weeks away from gifting the Map to Harry, saying they had already learned everything they could from it. They weren’t as attentive to the map, or most likely saw it as a tool to explore secret passages, not to keep track on those coming and going.

Plus, they give the map to Harry in an empty classroom - not the Gryffindor tower; Pettigrew would have no way of knowing the map had changed hands. Harry used the map once, outside Gryffindor tower, to go to Hogsmeade; Harry's second time using the map, to go to Hogsmeade once again, happened after Pettigrew/Scabbers had fake his own death to get out of there.

Bottom-line is: Pettigrew's name appeared in the map, and he was aware of it; also aware of the map's abilities, made sure to explore beyond the Gryffindor's common room and dormitories so his name wouldn't stand out to the twins. There are assumptions that he slept in Ron's bed all those nights, but those aren't confirmed in the first two books. He is only comfortable and lying about in the Christmas breaks, when the twins wouldn't have much use for the map if attempting to sneak out.