Upon hearing the sound from behind, Boss Ke was initially watching Doctor Qi and the others feed medication to An Xiaomian. When the woman being medicated called out the name Shangguan Xiu, Boss Ke was greatly shocked. He looked in surprise at the mentally unstable woman being medicated, wondering who she was and how she knew his master.
Boss Ke and his group actually did not know An Xiaomian. Previously, they knew about Shangguan Xiu because they discovered that Arthur was suitable as a test subject at Madam Feng and Lao Shan's place, but felt Arthur was too young and needed a few more years. So they investigated and found that Arthur's father was also perfect as a test subject, and then they devised a plan to capture Shangguan Xiu.
But regarding An Xiaomian, Boss Ke and his group did not investigate, hence they did not know her.
