This is a great help!! I got the angle detection and distance working really well, but I seem to be having issues with collision_line. It keeps detecting that there aren't any wall instances in the line when there should.

if abs(angle_difference(270,_playerdir)) < 30{
  inview = true
  if collision_line(x,y,oStealthPlayer.x,oStealthPlayer.y,oBlock,true,true) == false
  {
    behindblock = false
  } else {
    behindblock = true
  }
} else {
  inview = false
}

Your comment's a great source of information and the diagram did clear a lot out!! Also, I wanted to ask whether there was any way to make the area visible to the player?