furtivement

Il a été vu en train de manger un morceau de gâteau furtivement.

He was seen surreptitiously eating a piece of cake.

Cet homme là-bas nous observe furtivement depuis un moment, le connais-tu ?

That man over there has been glancing at us furtively for a while, do you know him?

Tom longea furtivement le couloir.

Tom walked quickly down the corridor.

Elle regarda furtivement dans la pièce.

She snuck into the room.