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 Les sentinelles de Saül, qui étaient à Guibea de Benjamin, virent que la multitude se dispersait et allait de côté et d'autre.