undetectable
1. [ adjective ] not easily seen
Synonyms: blind
Related terms: invisible
2. [ adjective ] barely able to be perceived
Synonyms: insensible indiscernible
Examples:

"the transition was almost indiscernible" "an almost insensible change"

Related terms: imperceptible
Similar spelling:   undetected
  undeterminable
  undeterred
  undatable