imperceptible

Imperceptible Adjective 1. V ery slight, gradual, or subtle. 2. N ot perceptible; not perceived by or affecting the senses.

Page 9 " At any rate Miss Baker’s lips fluttered, she nodded at me almost imperceptibly and then quickly tipped her head back again—"

"The nearly imperceptible sigh was enough to say how he felt about the class"

Ninjas hide, They're imperceptible.