Synonyms for Undetectable (same or very similar meaning)
WordNet sense 1 (impossible or nearly impossible to see; imperceptible by the eye):
invisible, unseeableWordNet sense 2 (barely able to be perceived):
indiscernible, insensibleWordNet sense 3 (impossible or difficult to perceive by the mind or senses):
imperceptible, unperceivableFrom the ODE community, based on WordNetadd/edit