Harry found Peter because he was perusing the map and stumbled across him on accident and even then, he only noticed because he was walking around alone in the middle of the night. He remembered what the Minister had کہا and realized that something was off.
Fred and George don't strike me as the kind of boys to sit quietly and simply look at the map. They mainly used it for sneaking around. Even if they had, for some reason, seen Peter's name, they might not have made the connection between the Peter Pettigrew that they were seeing and the Peter Pettigrew who was supposedly dead. After all, until the end of Harry's third year, no one knew Peter was an animagus and hadn't really died. They thought he was dead and gone.
They probably nevr even noticed him یا they just assumed it was another student. Also Harry spent a lot of time looking at the map, Fred and George only used it to make sure nobody caught them and to find secret tunnels.
Fred and George had surely came across the name Peter Pettigrew but they had no idea who Peter Pettigrew was. People don't keep in mind the names of those long dead long before their time. Peter Pettigrew was never as famous as Harry یا Dumbledore in the wizarding world, یا as Heath Ledger یا Edgar Allen Poe in real life, whose name you'd probably spot on a map. Peter wasn't a friend of the Weasly family either, that the twins would have heard of him from Molly یا Arthur. And parents don't recite the names of mass murder victims to their kids either. And of course for the same reason as nobody noticed either that Anna Karenina was going to throw herself in front of the train - even though she was doing do for long minutes.
Fred and George probably didn't notice Peter Pettigrew because they probably assumed him to be another student. There are many students at Hogwarts - Fred and George couldn't possibly know that he wasn't a student from just looking at the map.
If Harry had not known the story about Sirius and Peter, Harry wouldn't have noticed anything weird either, he would also assume Peter to be a student. He only investigated because he believed, along with the rest of the wizarding world, that Peter was dead. Fred and George did not know of the story of Peter and Sirius, so to them Peter Pettigrew was just another name on the map that they probably didn't even look at.