One-Man Scouting Party is a Fallout 3 achievement on Xbox 360 and PC and a trophy on PlayStation 3. It is granted for discovering 100 locations. You must literally go to the 100 locations, the Explorer perk puts all locations on your map but it does not discover them for you, once you travel to the location, the symbol on your Pip-Boy will change from File:Icon unmarked.png and it will be considered discovered, thus allowing you to fast travel.