What word is missing?
hidden
concealed
imperceptible
inconspicuous
indiscernible
obscured
unseen
shrouded
unnoticeable
unseeable
cloaked
imperceivable
undetectable
unnoticed
unobserved
clouded
discreet
disguised
indistinct
surreptitious
unexposed
unobtrusive
camouflaged
eclipsed
inappreciable
masked
nonexistent
obscure
secret
shadowy
unapparent
undercover
undetected
unperceivable
unrevealed
veiled
covered
covert
ethereal
gaseous
ghostly
imaginary
impalpable
imponderable
insensible
intangible
latent
perdu
screened
undisclosed
unobservable
vaporous