out of the corner of one's eye
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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.