In fact, no one knew that Wu Chengyue had broken into level-eight. He didn't even tell Xiao Yunlong about that.
When Lin Qiao told him that the man who attacked Yan Fangyu was at level-eight a couple of days ago, he wasn't surprised. He had that guess, but had no way of confirming it.
The day he arrived at the Upper City Base, he had sensed a level-eight vibe for a moment. It disappeared very soon, and it was from outside Upper City District. Therefore, even though he noticed it at first, he never sensed it again.
Clearly, the owner of that vibe had hidden it.
When he heard from the female zombie that Yan Fangyu and the two young men might have had an accident on their way back to the base, he started thinking about that vibe.