Is it correct to say that: "trifft auf" is obviously not a separably-prefixed verb (ie "auftriffen" (?)) because the prefix would have appeared at the end of the clause if it were?
treffen = to meet (also: to hit)
auf jemanden treffen = to come across someone
Ok, so "treffen" has an idiomatic usage when used with "auf." But "auf" is not a separated prefix from "-treffen."
Correct, ‘auf’ is a preposition here, not a verb particle.