A sentry is someone who is guarding a place, and they typically stand still and do not move energetically
Consider the statement "I looked to my right and saw a figure standing on the platform, holding a rifle and staring down at us energetically.". Is the statement "I looked to my right, toward the platform, and saw standing there a solitary figure, a rifle cradled in his arms, staring down at us like a sentry on the watch." false? Yes/no/unknown