arcanum
mysteryless
clouded
mum
crypto-
submystery
semicryptic
supersecret
secretomal
semisecretive
enigmalike
occulted
something nasty in the woodshed
recondite
mind-boggler
puzzler
under the table
riddle wrapped up in an enigma
pseudosecret
private
mystery
arcane
anyone's guess
ectocrine
enigmatic
secretly
backstairs
clandestinely
boggler
inenubilable
occult
underwrapped
head-scratcher
enigmatical
on the down-low
under the covers
dramystery
eye-opener
divulgation
skeleton in the cupboard
skeleton in the closet
problem
unrevealed
underground
mysteried
concealedly
penetralia
microclue
cryptical
crypto