[Quinn just said she went to Valmone after you left, and if she visited the ruins of Cruotis Prime or the planets in the Abyss Labyrinth where you, Jamie, and Evan trained, then Starbreaker likely noticed residual traces of Lady Celestia's energy signature.]
Those 'traces' were left behind after you used Lady Celestia in battle, and they're something only we Rule Breakers can detect easily. I tried to obscure them as much as possible with my destruction back then, but Starbreaker is a Single Digit Rule Breaker, so there's no doubt he saw through my attempts to hide it.
Now, upon coming to this universe, he headed straight to a planet named Athion. Considering the circumstances, the odds of this planet being a different one that coincidentally has the same name as the one where you retrieved Lady Celestia's fragment are extremely low.]
Once Lostvayne explained this, Arthur and Celestia immediately understood why Lostvayne had concluded that Starbreaker was after Celestia.
