So if I am wrapping my head around this correctly the sensor is pulling the pin low when it is clear and then letting the pin float when it is obstructed making an erattic reading. So by forcing the pin high with INPUT_PULLUP that makes the sensor act as a voltage divider. When the sensor is clear it drives the pin low actively by shorting to ground. When the sensor is blocked it no longer is shorted to ground and then the INPUT_PULLUP takes over and drives the pin HIGH. Well done gents thanks for the clarification. So the true answer is yes you ARE making a noob mistake. 
Thanks for your assistance.
amachinetech