If it weren't for the prior knowledge gained through the Sin Radar, Su Xuan probably wouldn't have recognized that this guy was a pickpocket. What made it even more awkward was that even if he knew the identities of these guys, he couldn't just take action in front of so many people.
Not to mention the sheer number of people here, but there were also multiple criminals. Even if he decided to act, he needed to know whether they had stolen any evidence from other people. If he approached and detained someone without evidence or them having committed any crime, it would be impossible.
The best approach was to wait for them to act, to catch them red-handed. That way, they would have the stolen items on them, making it a clear case of catching them in the act.
Sighing at the situation, Su Xuan refrained from taking direct action. Instead, he pretended to be like the other passengers, idly scrolling on his phone, occasionally glancing outside.