How does Athreos, God of Passage respond to creature tokens?
Neither of these scenarios happen. What does happen is called out in a ruling on Athreos:


  4/26/2014: Athreos’s last ability will trigger if a token creature you own dies. The target opponent has the option to pay 3 life, although the token can’t return to your hand.


The ability triggers as normal, since a creature died, even if it's a token. Your opponent is still given the choice to pay 3 life or pay nothing &mdash; no choice is made "automatically".

However, tokens cease to exist after they leave the battlefield. Even if your opponent declines to pay 3 life, the token won't ever enter your hand - it's already gone to the graveyard and will cease to exist there. So the result is the same either way: the token disappears. Naturally, your opponent is likely to just not pay the cost.