Originally Posted by cheerful
I have a 9300. I don't think it has that sensor. And it blinks when I use a headset.
It does have that sensor. All devices do. The sensor blinks much faster than the LED, and it will blink when you use a headset because it's trying to determine if it can turn off the screen.
As a test, make a call on your headset, manually lock the screen so that it is not lit, and see if you are still seeing a light.