In the process of moving about some area in order to observe, inspect, or protect it. There are only two guards on patrol, so there should be plenty of opportunities to slip in undetected. Officers Smith and McCluskey, you'll be on patrol in the east district tonight.
(out) on patrol
away from a central location, watching over a distant or assigned area. (Said especially of police and soldiers.) Officer Smith is out on patrol and cannot see you now. The soldiers who are on patrol on this snowy night must be very cold.