You know, with the way it's set up in the time line of the RP, Sebastian probably had a pretty big role in taking care of Shiori when she was younger, until her parents took her away from the manor. Yet when Ernest and Erika died, Shiori was assumed dead with them. Shiori had to have had a basic knowledge of demons thanks to her earlier childhood, so, she more than likely purposefully summoned a demon to get her out of captivity. Sebastian, whom, reminder, after the queen was taken care of, was no longer in a legitimate contract. He probably could sense it was Shiori (even if he thought she was dead), therefor, he was the one who came to save her. It also must've been a sweet reunion for the two as well, Sebastian would be glad because he's always been heavily attached to the Phantomhive family, then Shiori would've just been glad to see a familiar face