OK... working and tested
Be sure to add my.enable_scan = on; and my.enable_detect = on; to player's action.

Code:

---

 
function sentry_turnto(angle)
{
	angle = ang(angle - MY.PAN);
	if (angle > 10) {temp = my._speed;}
	else{
		if(angle < -10){temp = -my._speed;}
		else{temp = my._speed * angle * 0.1;}
		}
	MY.PAN += temp * time_step *2; //playe with this value to turn faster/slower
}

function sentry_event
{
	if(event_type == event_detect) &&(you == player)
	{
		vec_diff(temp,player.pos,my.pos);
		result = vec_to_angle(my_angle,temp);
		sentry_turnto(my_angle.PAN);
	}
}

function sentry_fun
{
	my.enable_scan = on;
	my.enable_detect = on;
	my._speed = 2;
	my.event = sentry_event;
	while(me)
	{
		c_scan(my.x,my.pan,vector(120,40,1000),SCAN_ENTS | SCAN_LIMIT | IGNORE_ME);
	wait(1);
	}
}
 

---

Cheers.

EDIT: Your sentry can pan around 360 dergees or should have some turn limit?
If yes I'll add something.
And for know it's only c_scan so I have to add trace, because c_scan working through walls.