invisible
unseeable
behind the curtain
behind the scenes
concealed
disguised
hidden
imperceptible
in disguise
incog
incognito
indiscernible
insensible
latent
masked
masquerading
out of sight
secret
sightless
submerged
unapparent
unbeheld
unbeholdable
undiscernible
unnoticed
unobserved
unperceivable
unperceived
unrealized
unseen
unviewed
unwitnessed
viewless