The three kids - Harry, Ron, and Hermione - are in the Shrieking Shack with Sirius Black when Professor Lupin appears, saying he spotted Peter Pettigrew (aka Wormtail or Scabbers) on the Marauder's Map which was left in his custody. Lupin says he saw Pettigrew with Ron, being dragged under the Whomping Willow by Sirius, on their way to the Shrieking Shack.
Peter, an animagus who transforms into a rat, has in that guise spent the last 13 years as a Weasley family pet and the last three of those years riding around in Ron's pocket. The wizarding world meanwhile has believed Pettigrew dead, seemingly murdered by Sirius Black 13 years earlier.
If Lupin can see Pettigrew on the Marauder's Map why hasn't Harry seen him as Harry has had the map since page 192, in other words for several months? It is impossible to imagine that Harry never looked to see where Ron is on the map. He would have seen Peter/Wormtail there beside Ron or in the dorm room Ron and Harry share with Neville and two other boys.
Lupin is one of the map's four cartographers. We are never told that he can see things on the map which other users cannot see, an easy enough fix for the problem. Oh, well...keeping all the details straight can't be easy.
