Lu Zhen didn't notice that Tawaki had actually fallen moments ago. He couldn't be blamed though. Ignoring the fact that Tawaki had died, Lu Zhen didn't even sense Tawaki's presence. Part of this of could be attributed to the fact that Tawaki was a couple of metres away and another reason would be that Tawaki's stealth abilities were superior to Lu Zhen's. This was understandable after all, Tawaki was one of the best [Assassins] in Emperor's Might while Lu Zhen was just a slightly better player than most. However, no matter how good Tawaki was, he still died while Lu Zhen was still alive, well for now that is.
Back when the kobold chief infuriated at Lu Zhen's stunt, both Sato's team and the God's Retaliation battalion heard the intimidating roar. Sato's group could easily guess that this was as a result of Lu Zhen's actions but the Blood beast's squad, who had recently arrived to the 10th floor, were left with expressions of confusion.