The Aliens could spawn from specific points and then pathfind to follow you. On Director's Cut it was essential to find the "between zones" the lay between the spawn volume triggers as the Pred to replenish your juice; Odobenus station was goddamn near impossible because it was so small the spawn triggers overlapped, resulting in about 6 aliens and two preds per 30 feet versus your Marine, and on top of that they gave you the SADAR to use in hallways about six feet across, which with blind and dead end turns meant you WERE going to kill yourself.