The Art of War by active 6th century B.C. Sunzi

17. If in the neighbourhood of your camp there should be any hilly

country, ponds surrounded by aquatic grass, hollow basins filled with reeds, or woods with thick undergrowth, they must be carefully routed out and searched; for these are places where men in ambush or insidious spies are likely to be lurking. [Chang Yu has the note: "We must also be on our guard against traitors who may lie in close covert, secretly spying out our weaknesses and overhearing our instructions."]