on patrol

(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.
See also: on, patrol
References in classic literature ?
Police boats are on patrol duty about us all the time, and it is as much as a sailor's liberty is worth to show himself in a red shirt.