able behind the curtain behind the scenes camouflaged concealed disguised enigmatic hidden imperceptible incalculable incognizable indiscernible insensible invisible latent mysterious out of sight puzzling sealed secret sightless strange submerged unapparent unapprehended unascertained unbeheld unbeholdable unbeknown uncharted unclassified unconsidered undescried undetected undiscernible undisclosed undiscoverable undiscovered undivulged unexplained unexplored unexposed unfamiliar unfathomed unheard unheard-of unheeded unidentified uninvestigated unknowable unknown unmarked unminded unmissed unnoted unnoticed unobserved unperceivable unplumbed unrealized unregarded unremarked unrevealed unseeable unseen unsolved unspied unsuspected unthought-of untouched untraced untracked unviewed unwitnessed viewless virgin