Watchmen or lookouts were a standard feature of war. You needed to know what your opponent was doing, where his troops were moving etc. Stationed on a hill between Gibeah and Michmash, the watchmen would have had a good view of the Philistine troop movements in the local area.1 Most likely they would have also heard the Philistines as they scattered.2
16 And the watchmen of Saul in Gibeah of Benjamin looked, and behold, the multitude was dispersing here and there.