out of the corner of one’s eye
Je l'observais du coin de l'œil.
I was watching him out of the corner of my eye.
Quelqu'un d'occupé qui surveille en même temps du coin de l'œil, c'est un flic.
Someone who’s busy but keeping an eye on things out of the corner of their eye is a cop.
— Infernal Affairs (2002)
Thomas l'observait du coin de l'œil.
Thomas was watching him out of the corner of his eye.
Il me regarda du coin de l'œil.
He looked at me out of the corner of his eye.