Pointers to Show Off-Screen Enemies

I have made this script to create a GUI Texture at every Enemy (/every tag you tell it to) and it works great so far but now I want to add an off screen indicator to show where enemies you can't see are. (sort of like this)

P.S- I dont need a real script... just "pointers" on how to do it

-heres my code-

 var multiplier : float;
 var style : GUIStyle;
 private var list : String;
 var range : int;
 var TargetTags : String[];
 var TargetTexture : Texture[];
 
 function OnGUI ()
 {
     for(var t = 0; t < TargetTags.Length; t ++)
     {
         var pointers : GameObject[];
         pointers = GameObject.FindGameObjectsWithTag(TargetTags[t]);
         
         list += "------------------ \n" + TargetTags[t] + "\n ------------------ \n";
         
         for(var pointer in pointers)
         {
             var dist = Vector3.Distance(transform.position,pointer.transform.position);
             if(dist < range)
             {
                 if(pointer.renderer.isVisible)
                 {
                     var size = multiplier / dist;
                     var rect : Vector3 = camera.WorldToScreenPoint(pointer.transform.position);
                     GUI.DrawTexture(Rect(rect.x -size/2,Screen.height - rect.y - size/2,size,size),TargetTexture[t],ScaleMode.StretchToFill,true,10.0f);
                 }
                 
                 list += pointer.name + "\n";
             }
         }
     }
     GUILayout.Box(list);
     list = "";
 }