The appearance of ambush assassins outside Liangzhou City indicated that the city had indeed been under surveillance all along.
Ling Hua braved the heavy snow to travel to Liangzhou, a journey few would likely consider, especially since she also had to pass through Youzhou, a difficult barrier. Even Wen Xingzhi might not have foreseen this, and the Ning Family of Biyun Mountain probably wouldn't have guessed either. Young Master Ning Ye should currently be at Ling Mountain; though it's not tens of thousands of miles from Liangzhou, it's still seven or eight thousand miles away.
The fact that one of the leaders had a Bamboo Leaf mark on the sole of his foot suggested that whoever bore this mark placed great importance on the assassination of Yan Qing. Upon spotting Yan Qing, there was no need to report to his master, he could strike immediately, and Yan Qing had to die. Otherwise, they wouldn't have mobilized so many people to assassinate him as soon as he left the city.