You can be as dumb as a rock(some are) to be a engineer, but the job generally takes a high IQ.
They do not design anything like a skilled engineer.(officers are degreed engineers for the most part) They do the heavy work. They drive the dozer's, clear the airstrips and landing zones, blow the obstacles with explosives and generally get dirty.(and shot at)
A sergeant normally has a squad of seven and does whatever task is assigned.(within his specialties)
This guy was on guard duty apparently and was most likely being disciplined or was in charge of other guards. I suppose the story will unfold.