When ‘He’ is in his spirit form he was able to see the flow of magic power after it had reached a certain point. Gems are able to absorb magic power easily, the gems and metal ores inside the dungeon are covered in the magic power it gives off, so they absorb it. It seems like the large rat that got mutated by the magic power of the dungeon, gets attracted to objects that have magic power within it.
“If it’s inside a monster’s body then it’ll become troublesome to find it.”