Watchman
A watchman is a soldier who guards a place, usually by standing at its entrance [Online Cambridge dictionary].
Sentry duty is commonly used throughout the Department of Defense (DOD) and Department of Energy (DOE) at several locations, including gates to military bases, aboard ships, along a fence or site perimeter, or in a hangar. Many sentry duty tasks support a security mission to prevent sabotage, protect property from damage or theft, prevent access to restricted areas by unauthorized persons, and protect personnel. The sentry is expected to devote full attention to whatever type of watch. The guard or lookout must maintain sufficient attention and scan a visual field for long periods to detect infrequent events [See, J., p. 9].
The soldier on sentry duty must not only detect the sudden appearance of a visual target but also pick up a rifle, aim, and fire accurately at the target [Johnson, R.F., p. 1414].
Sentry. Online Cambridge dictionary. Retrieved from: https://dictionary.cambridge.org/dictionary/english/sentry
Johnson, R.F. and McMenemy, D.J. (1989). Target detection, rifle marksmanship, and mood during three hours of simulated sentry duty. Proceedings of the Human Factors Society Annual Meeting, 33(20), pp. 1414–1418 (p. 1414). Retrieved from: https://doi.org/
See, J. (2014). Vigilance: A review of the literature and applications to sentry duty. Office of Scientific and Technical Information, pp. 1–88 (p. 9). Retrieved from: https://doi.org/10.2172/1322275