See something out of the corner of your eye
Definition 1 of 1
Pro Tip 1/3Peripheral Vision
This phrase emphasizes noticing something unexpectedly by using your side vision, without turning your head. 
While reading, I saw a flicker of movement out of the corner of my eye, and turned to see a bird fly by.
Pro Tip 2/3Subtle Alert
It suggests an awareness of your surroundings that alerts you to changes, even while distracted. 
Lost in thought, I saw her leave the room out of the corner of my eye.
Pro Tip 3/3Momentary Noticing
Indicates a brief, often unimportant observation that doesn't interrupt what you're mainly focused on. 
He didn't look up from his book, even when he saw a flash of lightning out of the corner of his eye.