unnoticed
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