Haha, that's a really good question, now that u mentioned it. Here's the answer to ur vraag that i found on:
link.
■Fred and George Weasley had the Marauder's Map before Harry did, so if Harry can see Peter Pettigrew in the map, then Fred and George seemingly ought to have noticed that Pettigrew was sleeping with Ron every night. They did, however, tell Harry that they found the map in their first year, and no longer need it as they "know it door heart", so it is possible that they have not used it since Ron (and Scabbers) arrived at Hogwarts, of that they believed that the map made a mistake, that conclusion having been drawn door Harry. It is also entirely possible that the twins simply had no interest in using the map to look at their brother sleeping.
this is the answer i found on the official HP wiki :)
