abstruse, arcane, back-door, backdoor, clandestine, classified, cryptic, furtive, hole-and-corner, inscrutable, mysterious, mystery, occult, phantom, private, privy, recondite, riddle,
secrecy, secret, secretive, sneaking, stealthy, subterranean, subterraneous, surreptitious, uncanny, undercover, underground, underhand