It's not only Taiyin Sect Master observing Ye Xuan, Ye Xuan is also observing Taiyin Sect Master, because Ye Xuan has never encountered someone at the Unity Realm Stage, nor does he know how big the gap is between him and the Unity Realm Stage.
However, when Ye Xuan saw Taiyin Sect Master for the first time, a smile curved at Ye Xuan's lips because the Taiyin Sect Master before him didn't exert much pressure on him. An intuition told Ye Xuan that the other party wouldn't be much stronger than him, and if he was at full combat power, he had a great chance to suppress them.
This is a kind of instinct, as well as intuition, like a tiger and a lamb meeting, even if neither knows what species the other is, that kind of bloodline suppression gives the most direct feeling.
