This ones been floating round for a while. Why don’t Fred and George ever notice Peter Pettigrew? Why doesn’t Harry for that matter.
But the twins more so because they have had two and a half years to notice that there’s an extra person sleeping with their brother. So what the hell?
Well I think there are a few things going on here. Firstly it might not be that clear on a map exactly how close people are. There’s only a certain amount of room for names so things will probably get nudged out of place a bit. Therefore it might not be clear Ron and Peter are IN the same bed. Just the same room. And how interested would Fred and George really be in Ron’s dorm mates. It’s not like ‘Peter Pettigrew’ would necessarily ring any bells for them. Though Gryffindor House is fairly small so they might think it was a little odd, for more on that do see my post on Numbers. The other things is that Fred and George are using the map to get around the castle and avoid trouble (ie. teachers, Filch, Mrs Norris) they probably just aren’t that interested in Ron’s sleeping arrangements. Why would they be looking at Gryffindor Tower at all? After there first year (before Ron even starts) they are probably only using the map to check for trouble. They’ve probably pretty much learnt the secret passages. So it’s open map, no Snape, do what you need to do, get out.
Lupin only notices because he is watching Harry, Ron and Hermione in case they sneak out. There are over a thousand people on the map. You aren’t going to be looking at all of them.