covert
able
arcane
cabalistic
censored
classified
close
closed
concealed
cryptic
dark
enigmatic
esoteric
hermetic
hidden
hush-hush
incalculable
incognizable
invisible
latent
mysterious
occult
puzzling
restricted
sealed
secret
smothered
stifled
strange
suppressed
top secret
ulterior
unapparent
unapprehended
unascertained
unbeknown
unbreatheable
uncharted
unclassified
under security
under wraps
undetected
undisclosable
undisclosed
undiscoverable
undiscovered
undivulgable
undivulged
unexplained
unexplored
unexposed
much-neglected
objectivist
thought
unattained
unattempted
uncollected
undescribed
undreamed-of
unexamined
unfathomed
unguessed
unimagined
unindexed
unissued
unrealised
unrealized
unreleased
unsolved
untapped
unthought-of
unutilized