Remember though that in both the book and the movie Lupin takes the map from Harry after he works out that Harry has been using it to get to Hogsmeade (in the movie he takes it the same night that Harry sneaks out of bed to chase Pettigrew down). In the book its the day of the Hogsmeade visit after Christmas, and there is still snow on the ground - because Harry throws snowballs at Draco in Hogsmeade that day. So, it must be still wintertime. Maybe Harry just wasn't looking for Pettigrew on the map. His main use for it was to use it to get to Hogsmeade with - mainly to make sure no one could see him.