keep watch
Also, keep a watch or close watch on; watch over. Observe with continuous attention, especially to act as a sentinel or for protection. For example, Afraid that the wolves would return, she kept watch while the others slept, or They kept a close watch on the harbor, looking for signs of enemy ships, or, according to the Gospel of St. Luke (2:8): "And there were in the same country shepherds ... keeping watch over their flock by night." [Late 1300s] Also see keep an eye out for.