Why couldn't Fred and George see Peter Pettigrew on The Marauder's Map?

 I'm sure this question must have been cropping up in our heads whenever we read Prisoner of Azkaban. I just want to put it out there, that I will be basing this off of the books rather than the movies.

So, first of all, how didn't Fred and George see Peter Pettigrew (a.k.a. Scabbers) with Ron all the time? Even when he fell asleep? Knowing the twins, they wouldn't miss a single opportunity to tease their younger brother. Many people suggest that Fred and George simply didn't notice it, but I don't think so. They have used the map uncountable times, it would be impossible to be so oblivious.

I found a really good theory by Seamus Gorman on YouTube, and it is more or less similar to mine. The theory is, that no one but the marauders can see themselves on the map. It is applicable, as Lupin was the first person to notice his old friend on it after he confiscated it from Harry.  This would have been pretty convenient for the gang in their school days, because even if someone got their hands on the map (and managed to decipher how to use it), he/she wouldn't be able to track the marauders at all! You gotta admit, the marauders may have been mischievous, but they were smarter than many of us.                                                      

Next up: How do you become Animagi? How did the Marauders manage it?