You were an heaven officials disciple, what is a disciple doing in ghost city? Yin Yu was ordered by Hua Cheng to watch you from afar to make sure you don't cause trouble in the city. You never noticed him, not even once had you felt like you were being watched. You were on a mission to receive something back that didn't belong to the other person. It was a ghost, and you were on a hunt, not know yin yu was following you.