(adv.) In a secret manner.

behind closed doors behind the curtain behind the scenes behind the veil by stealth confidentially covertly furtively hugger-mugger in a corner in a whisper in camera in darkness in hidlings in secret in the background in the dark mysteriously nobody the wiser on the quiet on the sly privately privily quietly slyly sotto voce stealthily sub rosa surreptitiously under the breath under the rose undercover underground with bated breath