Luckily, they have an S rank porter. I didn't expect him to be able to pull off such stealth, though, that only one member of the actual squad really noticed it.
@Kaarme Since he is more experienced in group fights, he can use his hiding skills more efficient than those who using skills recklessly.
Experience is your best friend when it comes to taking decision and using suitable skills for each problem faced.
@hartex92 How is he more experienced in group fights? The very name of this series is Solo Leveling...
This squad he's working as a porter for is a fully functional group with the members totally depending on each others. They are the ones experienced in group fights and would be naturally inclined to try to keep everything on an eye as their teamwork depends on that.
@Kaarme Thats the power of S rank stealth. Its been shown S ranks are just that much better after all. To be fair if he had min maxed into a true S rank assassin they likely would have noticed nothing but the effects of what he did. Still any assassin type worth their salt could probably avoid peoples gaze and cut enemies to poison them. The groups were both focused on fighting each other after all.