Actually, Zhao Yu had not find the culprit by using his wits. He had been able to determine that Li Dan was hiding in the grave by using the Miracle Systems’s gift, the invisible tracking device!
When Zhao Yu had been shot with the anaesthetic needle, at the verge of unconsciousness, he suddenly remembered the invisible tracking device and planted it on Li Dan using his last bit of strength. This tracking device was indeed powerful. It was totally invisible, and Li Dan had not discovered it at all. When Zhao Yu woke up, he quickly became aware that there was a radar in his brain, and that the target on the radar was pointing at Li Dan’s location, thus that scene in which he found her had taken place.
Zhao Yu had not expected Li Dan to be so cunning as to dig a hole beforehand and leave an escape route for herself. Although the grave was empty, what kind of mental torture would it be to lie inside and not speak for days? Any ordinary person would not able to do that.