ἀνεξιχνίαστος - "untraceable"
Adjective derived from ἀν- (alpha-privative) + verb ἐξιχνίαζω “I trace out”, from ἐκ “out of” + ἰχνιἀζω “I trace”
The substantive form of ἰχνιἀζω is ἴχνος, “track or footstep”. Hence the verb ἐξιχνίαζω is to follow the tracks of something, as of an animal in the woods, or an argument to be grasped. The adjective form ἐξιχνίαστος is thus, “able to be tracked, traceable.” The alpha-privative prefix negates this, hence “unable to be tracked, untraceable” as are the ways of God, and the riches of Christ!
Examples: Romans 11.33, Ephesians 3.8