out of the corner of one's eye - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Glancing casually or surreptitiously, as in Out of the corner of my eye I saw Justin walking out the door. This expression refers to looking sideways at something rather than directly.