To really understand everything though, let’s take a brief overview of what the map actually is, its history and a few side notes on the parchment paper. The first thing to point out is that the MM shows all of Hogwarts and its grounds within the confines of ink and paper. When in use this map is absolutely massive, coming in at a whopping 15.5x72 inches (39.37x182.88cm for the wrong crowd). That is over 1,116 square inches of parchment to look over! It shows every hallway, classroom, meter of the ground along with labeling people as dots. It also shows real time movements, which is important when you want to sneak into Honeydukes, past a certain past owner of the map to fill your pockets with candy. The only things that are not shown on this map are (for certain) unplottable rooms, like the Room of Requirement and (theoretically) undiscovered rooms, ala Chamber of Secrets (though this could be unplottable as well). It also, and this part is very important, never lies. It will reveal those hidden by magic (polyjuice potion and invisibility cloaks), it will show ghosts, and even animagi are shown by their true names. Well there Brian, you just defeated yourself! You just admitted that it would have shown Pettigrew! Hold your Prongs, we’ll get to that in a little bit.
So as we’ve seen the MM is a powerful piece of magic, containing a lot of knowledge and it doesn’t lie. The creators made it with all of this in mind. Now to keep this short, I won’t go into detail about the creators in my written explanation since I am trying to make a deadline, but suffice to say these dudes were relatively smart and talented. This next part will go into the groups who owned it and why each one wouldn’t have really noticed Peter.
Let’s knock off the first owner that we learn of Filch. As far as we know, he never learned how to use it, which means this portion is done, unless we want to go into wild speculation. However we can infer with relative confidence that since he did NOT know all of the secret passages he did not know how to use it. Now let’s move onto the twins. Fred and George (He’s not Fred I am,) are the next owners after they liberated it from its lock up in a filthy room (pilfered if from Filch). So first things first, when the twins got the map, it took time to learn to use it, which puts a dampener on seeing Wormtail if he was even there with Percy (original owner) at all. After that they would have used all of their time to memorize the map, every detail, passage, password and girls dorm they could have, not focusing on the Gryffindor common room after the initial memorization. Why not, you may be asking yourself. There would be no need, since most of their headaches were either Filch, or other teachers getting in their way of sneaking about. That coupled with the fact that Filch would probably follow unique patters and we had rarely heard of Wormtail leaving the dorm rooms means the twins were almost the least likely to notice him.
Now Harry is a different story, when he receives the map, wormtail is missing and Harry himself didn’t really even know his name, so why would he look in the dorm room. But Brian, you persist, he should have recognized that Peter Pettigrew was a name he didn’t recognize on the grounds if he ever saw it. Suffice to say, you may be right! If we go by the movie assumption of how many students are in Hogwarts. Again, to save time we won’t do the math, or calculations it’s just safe to say there are a lot of kids there, did you know everyone in school when you were younger?
So there you have it, a reasonable theory, that was lightly fleshed out in the matter of 30 minutes as to why no one noticed Wormtail in harry potter.