Marauder's map question, prior to PoA

[Robyn]

No: HPFGUIDX 191946

Hi,
Why didn't Fred and George ever notice Peter Pettigrew on the Marauder's map prior to PoA?  Why would they not have noticed that someone named Pettigrew was very often in the same place as their brother Ron, or at least, that someone they didn't know was hanging out in the Gryffindor common room and dormitories? I have looked at the FAQs on this topic, and feel like there must be more to this ... is this a hole in the plot?
Thanks,
Robyn