Currently Zero is hiding himself while in front of him, you can see many monsters hanging upside down in the ceiling of that underground Dungeon. Currently Zero is on the second floor and he is already able to meet those intermediate Mortal-grade monsters.
Those Monsters are the bat Monsters and like those rat Monsters, they also have a very big number. But it is just their number did not increase as rapidly as those rat Monsters so the guardian association did not provide missions related to those Monsters.
Zero did not care about that as he wanted to kill every Monster to gain exp. He did not want to attack those Monsters recklessly without knowing the power of those Monsters. On the previous floor he just only met those rat Monsters and only a few spider Monsters.
After coming to this new floor he is finally able to meet those bat Monsters and by looking at those Monsters he can tell those Monsters most likely have the same power level as levels 5 to 6.