Owen Bertram looked serious. The presence of the little girl was quite telling: Customs City was seriously riddled with vulnerabilities. Even though the Demon Clan has retreated, they should still be cautious of potential spies lurking within the city.
Customs City, which was recently conquered by the Human Race, had yet to develop an Array specifically designed to screen the Demon Clan. Moreover, there are over hundreds of thousands of slaves living within Customs city, among whom are undoubtedly those who've pledged their undying loyalty to the Demon Clan. In a very short period of time, Owen Bertram managed to contemplate various strategies and decided to immediately order agents from the Horizon Bureau to enter the city. Counter-espionage operations needed to start without delay.
Another aspect was defense. During the retreat of the Demon Clan, it was the best opportunity for the Horizon Bureau to plant spies among them.