Fei Yinzi's eyes are wide as he stares at Fei Yin who is leaning on a boulder with his arms crossed over his chest. If Fei Yin didn't speak, Fei Yinzi wouldn't know that Fei Yin was also here! Fei Yinzi didn't notice Fei Yin. If Fei Yin wanted to hide his presence, Fei Yin could and even could spy on him (FYZ) while he voices out his thoughts on the peak of the mountain which is very relaxing, prompting one to release their grievances. And so Fei Yin definitely could have heard Fei Yinzi secretly complaining here about him without Fei Yinzi knowing.