spot the difference (puzzle)

Exploration Stage: Stargazing Terrace
From: Kira Woodville
Description: There's no sign of anyone on the Astronomy terrace, but I hear footsteps coming from that direction. Then the story is completely different. The "shadow" disciples don't have any substance, so they don't make any noise. If they're human, it's all right. Let's go!
Completed: This terrace looks the same as it did before the fog invaded the city. But if you look closely, there are a few places that have changed. It seems that someone here noticed us and ran away. Let's take a closer look around. I'll report the current situation to the director.