Or it categories magic stones differently for different classes of monsters. Small medium large kind of thing. Thats fairly common in these fantasy stories where monsters have stones.
We were informed that magic stones are present in monsters, and he's been able to avoid all monsters when using his detection skill on magic stones. Maybe he could specify his detection by size or by magic stone of a specific species of monster, but he's never done that and instead always generically searched for magic stones. The only time a monster snuck up on him was because he had forgotten to use his detection skill.
For instance, when he was searching for the flowers with 3 petals instead of 4, he just searched for the type of flower and it found them all regardless of the difference in the number of petals. Instead another part of his skill, which indicated the rarity of the item, allowed him to quickly find the location of flowers with 3 petals as they were rarer.
It hasn't been shown because he's spent all his time avoiding monsters, but my expectation is that his detection picks up on the location of all magic stones, and depending on how big or how much magic they have stored, they'd be rarer. I would be very surprised if this monster has a magic stone and his detection failed to find it.
If it is indeed the case that the monster has a magic stone, then I expect some explanation regarding his detection skill's failure to find the stone. Perhaps the monster is still currently out of range or perhaps the monster uses magic which allows it to block detection or perhaps it has something that is no longer considered a magic stone but still serves the same purpose of supplying magic power to the monster (maybe a magic gem instead of stone, and the difference is name would be enough to prevent the search function from finding it).
The monster seems like a reptile of some sort so perhaps a dragon. Maybe it has scales which reflect magic.