If the first half of the marching chapter dealt with where to settle, the second half deals with how to read the other side.
Sun Tzu says that without any large network of agents, you can read their intent from small movements in front of you.
眾樹動者，來也；鳥起者，伏也；獸駭者，覆也
zhong shu dong zhe, lai ye; niao qi zhe, fu ye; shou hai zhe, fu ye
Trees stirring means they are coming. Birds rising means an ambush. Animals bolting in fright means a surprise attack is hidden. (The Art of War, On the March, translated directly)