The Marauders' Map did not have the COS on it, because the Marauders did not knew where it was or it's entrance.

__________________
There is always time to read. Read as much as time as possible. To be ahead is to live. Think before doing anything.



Have Parties.

The Map only shows what the makers had discovered. There were places that knew nothing of, one being the CoS, another would the Secret Room where the DA met, Ma;foy did his secret work (also I believe that this place would be unplottable because of the rooms special powers. Also, it did noy have the secret passage from the Hoshead to the Room of Requirements on DH.These are but a few, I expect that there were many places that were sectret and still remain secrert to this day!