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