How does he know?
Si Ning had nobody to reply to the question he just asked in his head. According to Xiu Zhicheng, it was the Queen Dowager that suggested half of the Si family army joining the Xiu army and there had never been a discussion like that in the court which means someone closer to Xiu Zhicheng or someone closer to the Queen Dowager gave the information to Qiu Bai.
Qiu Bai telling him about it means he was confident that Si Ning would be joining Xilie or Si Ning would never tell a soul because they were meeting in secret or it was Qiu Bai means of letting him know someone was working for him close to the Emperor, Queen Dowager of a member of the Imperial family because he considered Si Ning not to be a threat.
Si Ning had already known someone was working close to Qiu Bai but he didn't try to find out the person because he never considered them to be deep in the inner court or someone who might be closer to Xiu Zhicheng or the Queen Dowager.
