You died a year ago, in the Battle of Manhattan. And Percy had grieved and mourned you and your absence since that day he failed to save you. And since Thanatos was abducted, and the doors of death were open, any dead person could slip back into the living world. He was just hoping you’d figure that out too, before they closed again. Because here he was, on a boat with Hazel and Frank, going to save Thanatos and close the doors of death. And ruin his last chance of seeing you again.