Tom would furtively read Mary's diary while she was having a bath.

She was deadly pale and breathing fast, glancing furtively towards the bed as she fastened her mantle, to see if she had disturbed me.

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

Tom looked around furtively.

Tom glanced furtively at Mary.

Tom furtively glanced at his watch.