Tbh I feel like the extragalactic threat in Gero's story in Hunt The Truth is the Flood probably comandeered what was left of the ship and used it to get somewhere, or crashed somewhere and are now a threat. It would make sense that they could've survived because there has been another Gravemind or two, and it hypothetically would've been in control of them until he was destroyed again. Idk, just a theory but since its so "under the wraps" and not widespread to the general public, it must be a pretty big problem if its in Deep Space and the SoF made a random jump.