White Cloud City's security wasn't as good as Qishurg City's. The White Cloud Sect was in charge of the city, after all. Only a few kingdom soldiers were inside the city. Their primary task was to protect the kingdom officials in the city hall, whose responsibility was mainly to receive reports from the White Cloud Sect.
Hence, there were no kingdom soldiers registering everyone who went in and out of the city. There was not even a city wall to prevent people from traveling in and out of the city as they wished. The Demon Gang and Vicious members entered the city inconspicuously. They stayed separated and roamed around different parts of the city to avoid detection. All the while waiting for Amon's signal.
As for Amon and his team, they were canvassing the Wei Clan Estate.
Amon had already conducted reconnaissance on the estate while he was living in White Cloud City. He did it again to ensure he was aware in case the landscapes changed during his absence.