With every fireball that drops, a large swath of demons is incinerated. Throughout the day, Du You doesn't even know how many demons he has slaughtered. When Du You's magic power can no longer persist, he commands his summoned creatures to start withdrawing.
Clearly, the density of demons in this large area from which Du You has withdrawn has significantly thinned.
Tomorrow, he'll go after another location. When he's out of magic power, he'll return to rest within the inner city walls. This is what all demon hunters in this world typically do.
Next, Du You daily hunts a batch and with continual hunting, he can't possibly avoid all the professionals. After all, professionals comprise a significant portion of those dealing with demons here.
Moreover, Du You every time makes such a big commotion, it's impossible that he wouldn't be noticed by others.
