The Yi family's main residence was in the capital of the Mystic Region. It was too far away. Unless Fan City's City Lord Residence could deliver relevant information swiftly, it was unlikely that the other party could find the Heaven Mending Barrier at the entrance of the Primitive Burial Ground so quickly.
To be more precise, two days after Ye Chen took away the secret treasure in the Li Residence's underground palace, there was still no one guarding Demon Flame Mountain. This was enough for him to raise two guesses, either the forces in Fan City, who were connected to those large families, didn't quickly report the various situations, or not every large family understood the relationship between Demon Flame Mountain and the array in the Li Residence's underground palace.