Twilight Mountain, mountain peak, arena.
"Did you notice anyone suspicious?" Qiu Shiqi asked Qiu Ba when he returned to the area for the Huichen as guests in the Summit Meeting. He is referring to the person who He Wenzhe and Qiu Liu were referring to who was secretly watching them.
Just the thought of this is unnerving and chilling, not to mention makes you feel uneasy. Since He Wenzhe and Qiu Liu are in the most precarious situation right now as they are at the front of their (Huichen's) 'battle' as they are impersonating Qiu Shiba and Meng Xing, respectively, having someone secretly watch them and even aware of it now makes them doubly tense and nervous, which would be detrimental for their task. It was already fortunate that they could hold it in until now, showing how great their mental fortitude is to handle such pressure and stress.