Once the horde has been thinned the Alpha Company will inform you that they believe the Infestors are the source of the infestation. They managed to capture one of the more docile ones and are keeping it at the Military Base. They also have taken a liking to it, and dubbed her Charlie. Unfortunately when they attempt to move her she becomes agitated and runs off. You must now find her as she runs about the map at high speeds. Charlie will generally be found running around the central area of the map, so its a good idea to check there first.