When the group left, Walker knew that Clara had also made a few other plans. He didn't say a word about it, but there was more than just the group he had gotten together leaving.
He used his mana sense to check the surroundings constantly. Whether he was in the city or outside of it. This was just something he did even without thinking now. More of a passive aspect of his skills rather than an active one at this point. It was all just him being prepared and also partially at his second sight now.
He could sense multiple people hiding themselves, or at least trying to. It only took him a second to realize that Clara had the assassin trainees who were trying to join and improve themselves within the adventurer's guild attempt to remain hidden around their group the entire way.