camouflaged, undetectable, infrared, occult, invisible, lightless, unseen, out of sight, nonvisual, ultraviolet, inconspicuous, concealed, hidden.
The instant Jack Cornwall saw those eyes, he knew that they could see almost unseeable
For myself I recall a chill of inward horror; a revulsion as though around me were pressing unknown things; unseeable,
imponderable things menacing us all.
She stood in that horrible inflexibility, with eyes like black stones fixed on something unseeable.