WordNet sense 1 (be difficult to detect or grasp by the mind):

subtle

WordNet sense 2 (imperceptible to the senses or the mind):

impalpable

WordNet sense 3 (impossible to identify):

unidentifiable

WordNet sense 4 (skillful at eluding capture):

evasive

WordNet sense 5 (marked by skill in achieving a desired end especially with cunning or craft):

artful