I Bet He's Thinking About Other Women

That's actually a pretty big plot hole. My guess is that there were too many people to notice Peter Pettigrew in the daytime. When Harry checked the map in the nighttime, he could see Peter's dot very distinctly since there are much less people roaming around at night.

Did Fred and George know who Peter Pettigrew was? Maybe they only looked where Ron was a few times on it and when they saw the name they just thought it was just a regular person who was near or talking to Ron.