What is one who stand on guard and sounds an alert if something is wrong?

One who stands on guard and sounds an alert if something is wrong is often referred to as a "sentinel" or "watchman." Their primary role is to monitor an area for any signs of danger or irregularity and to notify others if a threat is detected. This responsibility is crucial in various contexts, such as military, security, or even in wildlife settings, where vigilance is necessary for safety.