Impossible to perceive.
‘his head moved in an almost imperceptible nod’
‘Yet the incursions on free speech can be insidious and imperceptible.’
‘Without museums, the skills of the sculptor and the various dialects of tribes in the country would have been imperceptible now, he said.’
‘He lived in a world of fine gradations and imperceptible shades.’
‘On these repeat visits, subtle qualities gradually reveal themselves which were initially almost imperceptible.’
‘I could hear the call going along the line, but then at the final moment, instead of ringing, there was a strange, almost imperceptible hiccup.’
незаметный, неощутимый, незначительный