I think he went to another reality like the exo stranger came to our reality, he probably wants to change something the nine did or what he did to start the collapse to find where the Traveler first came from. I think the traveler rebelled against the witness because the witness made the traveler and the traveler did not want to be a part of the witnesses plan and fled to find a race to fight the witness, or the nine made the traveler to find a race worthy of the light power to fight the witness and the witness gave us darkness power to subpress the light power, I would say the witness is trying to find a traveler made of darkness to destroy the nine and the light power they have, plus the nine did not agree with what the witness did so they expelled him from there realm and he is trying to find away back to the nines realm to destroy them, also the witness is both light and darkness just like the nine, but the nine favor the light over the darkness, the witness favors the darkness over the light, also the light artifact that we destroyed on neomuna the witness probably stole it from the nine to pass between the veil and the veil is just a divider. Plus the nine gave the drifter that gift that's on his ship on top of that the nine seem to be quiet but come back soon to the story in the last expansion of the game.