out of the corner of eye
out (of) the corner of (one's) eye
Of something seen, in one's periphery; partially and/or fleetingly, as opposed to fully and for an extended amount of time. Out the corner of my eye, I saw something moving in the bushes, and it turned out to be a stray cat. Sarah caught Tommy kissing Jessica out of the corner of her eye.
out of the corner of one's eye
Fig. [seeing something] at a glance; glimpsing something, as with peripheral vision. I saw someone do it out of the corner of my eye. It might have been Jane who did it. I only saw the accident out of the corner of my eye. I don't know who is at fault.