a person who keeps guard over a building at night, to protect it from fire, vandals, or thieves.

(formerly) a person who guards or patrols the streets at night.

noun

caretaker , curator , custodian , detective , flagman , guard , keeper , lookout , observer , patrol , patrolman , picket , policeman , ranger , scout , security guard , security man , sentinel , sentry , signalman , spotter , spy , ward , warden , watch , watcher