Secret

abstruse ambiguous arcane backdoor camouflaged classified cloak-and-dagger close closet clouded conspiratorial covered covert cryptic dark deep disguised enigmatical esoteric furtive hush-hush mysterious mystic mystical obscure occult on the QT out-of-the-way private recondite reticent retired secluded shrouded strange undercover underground under wraps undisclosed unenlightened unfrequented unintelligible unknown unpublished unseen veiled

backdoor backstairs camouflaged classified close confidential covert cryptic discreet disguised dissembled dissimulated furtive hush-hush in ambush incognito inside restricted secretive sly sneak sneaky stealthy sub-rosa surreptitious top secret unacknowledged under false pretense underhanded under-the-table