behind someone's back
unbeknownst
unbeknown
undisclosedly
occult
off-the-record
on the hush
arcane
under the rose
under the table
privily
unknownly
in secret
concealedly
live a lie
on the down-low
supersecret
covertly
under the covers
underground
hugger-mugger
semisecret
latently
closehanded
secretly
backbiting
off the grid
something nasty in the woodshed
self-knowing
give away
unknowing
sneaking
secernent
unknowledgeably
clammish
for all one knows
close
secretive
undisclosing
have the advantage of someone
semisecretly
semisecrecy
closelipped
behind the scenes
holocryptic
closemouthed
get wind of
tightlipped
sieve
unsecretly