When magic is used, the mana residue lingers for some time.
One could sense the mana residue in the air less than an hour after the magic is used. Meanwhile, on a solid surface like the earth, gravel, concrete and such, two to three hours is the time frame.
The Hunter Association agents and the police are using this method in profiling the awakener at a crime scene. It was an indispensable skill that was often seen during investigations.
Naturally, after working in the Hunter Association for a while, Shen Yuan was able to pick up the sensory know-how.
He knows how to use the gadgets for measurements used to read and record the mana residue. He became familiar with the process to the point that he could make a step-by-step manual.
However, Shen Yuan is not Shen Yuan if he didn't come up with shortcuts.
