behind the scenes
backstage
on the hush
on the QT
in secret
secretly
on the Q.T.
confidentiality
concealedly
sub rosa
under the covers
on the down-low
down low
off-the-record
under the rose
secretively
closehanded
close-tiled
clandestinely
closeness
cryptadia
darkness
hugger-mugger
semisecret
privily
hidness
cloakedly
under wraps
shoot off at the mouth
semisecrecy
keep a lid on
secret
keep from
clandescent
underhanded
concealment
divulgence
divulgement
air one's dirty laundry in public
conceal
out
under one's hat
crypto-
mum
subterranean
secernent
private
leak out
pseudosecret
in petto
occulted
underhand
hidingly
something nasty in the woodshed
under the table
secretive
covert
undercover
closehandedness
unconceal
in camera
hole-and-corner
hiddenly
between you and me
confidential
hush up
semisecretly
confidence
off the grid
undisclosedly
hush-hush
hiss-and-tell
back street
exposure
disembosom
hiding
make known
covertly
top secret
supersecret
sensitive
underwrapped
behind closed doors
offstage
on a need-to-know basis
sneaking
hiddenness
covertness
stealth
doc drop
soap