Lin Ze knew then that Jiang Xu was behaving strangely, which was the first time that he had seen him do so. Instead of being wary or careful toward the enemy who could kill all of them that time without a single sweat, Jiang Xu instead kept provoking Zi Se. Even if Jiang Xu has a handle* on Zi Se, which is the Caose Kingdom's prince, yet Zi Se could have used anyone present as a handle against Jiang Xu to coerce Jiang Xu into telling him (ZS) where their prince is.
*something that someone could use against another person
Thus, Lin Ze felt that there was something strange between Jiang Xu and Zi Se, especially when Zi Se didn't even retaliate against Jiang Xu all this time despite how Jiang Xu antagonized him, no matter how irrational it was.
"I do." Zi Se honestly answered. His expression didn't even change.