James and Remus worked hard on perfecting the map over the next few days. Remus spent all his free time on it, eventually figuring out how to get everyone's names next to little dots that showed where they were. James knew the castle best, so he worked on including every room, shortcut, closet and secret passage to the map. Sirius had the idea that the map should spell out insults to anyone who tried to read it without the correct unlocking-phrase, while Peter, being neither academic nor hard-working, tried to be encouraging.
It was the second week of the new school year when the map was finally completed. It showed everything that any adventurous student wanted: who was where, all the hiding places, and every known (or unknown, to most people) path from Hogwarts to Hogsmeade.
Late one night in the dormitory, Sirius was idly following the map's little dots to see who was out of bed.
"Hey, Prongs? Moony?" he called, "I don't think this map works properly, we need to fix it."