Imperceptibility is a noun for the quality of being so small, slow, or quiet that people almost cannot see it or notice it. You will find it in careful, often scientific writing, usually about change that does not announce itself: the imperceptibility of growing older, of a slow leak, of a shift in the weather. The thing is happening; it just sits under the edge of what anyone would notice. Though actually, most English speakers never say this noun out loud. They use the adjective imperceptible, or they just say you could barely tell. Keep this word for reading. If you need to speak, a simpler path is almost always kinder.

難以察覺