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Lin Chen was able to detect the lingering Yin Sha Qi thanks to his cultivation of the Primordial Transformation Art, which allowed him to see the not fully dissipated Yin Sha Qi on Lu Feiyun's body.
That is to say, before Lu Feiyun returned to the sect, he had actually been afflicted with the Yin Sha Poison, but later managed to resolve it himself.
Lin Chen suspected that the people of the Black Cloud Sect might not have known that Lu Feiyun was their spy within Jade Cauldron Sect when they initially clashed, and only after they had poisoned him and he explained himself did they realize his identity and subsequently cure him.
All this became the key reason for Lin Chen's suspicion towards Lu Feiyun.
"I see, but the reasons you've mentioned are only enough for you to suspect Senior Brother Lu, so how does Senior Brother Bai also know about it?"
Zhao Rushi asked, puzzled.
Upon hearing this question, Lin Chen simply shook his head.