Locke was the one who saw the map on the blast door. Locke was the one who found the Pearl station (with Eko's help). The Tailies were found in the Arrow (did they NEVER discuss where they had been living..uh.. no.. wait... they did because Eko had the piece of film that Locke had seen). Locke had seen the blast door map, and then finds a station with a whole bunch of tv monitering screens - he knew there were more projects.
It doesn't matter now, but why did Locke not try and get Sayid to flip the black lights in the hatch to show others the map?