How did MWPP make the Maunderer's Map?

[Hagrid]

No: HPFGUIDX 154175

The Map has blind spots where it can't see anyone. So how come some 
areas show up and not others?

The three areas not shown on the map I know of are:-

- Hagrid's Hut (or Lupin would have seen Pettigrew there - POA)
- Room of Requirements (where Draco disappeared off the map - HBP)
- The Chamber of Secrets has never been mentioned again.

Do the makers of the map have to have been in those areas of Hogwarts 
to be able to include them on the map? 

- If so, Hagrid didn't trust them enough to allow them into his hut.
- They never discovered the secrets of the Room of Requirements.
- They couldn't open the Chamber - just as well for all of Hogwarts.

- BUT ... it also means they got into the Slytherin Common Rooom to 
plot that on the map because Harry sees Draco on the map when he is 
back in the Common room(HBP).

Agree? / Disagree? / what would MWPP be doing there? Was it Pettigrew 
(as a Rat) that crept around plotting some areas? Would his map making 
skill come back into book 7? What do u think?