invisible, unseeable - impossible or nearly impossible to see; imperceptible by the eye; "the invisible man"; "invisible rays"; "an invisible hinge"; "invisible mending"
2.
undetectable - barely able to be perceived; "the transition was almost indiscernible"; "an almost insensible change"
imperceptible, unperceivable - impossible or difficult to perceive by the mind or senses; "an imperceptible drop in temperature"; "an imperceptible nod"; "color is unperceivable to the touch"
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