To be more precise, explorers use Dungeon Walk and adventurers use Field Walk, and Michio is probably the only one who can use teleportation.
During his mission with the elves, he must have made people believe he was using Field Walk, but this spell is only used by adventurers.
To become an adventurer, Michio must become a level 50 explorer, thus unlocking the adventurer profession and gaining access to the Field Walk skill.
When you become a level 1 adventurer, your ID card will display this profession, and no longer the explorer profession. As you can see on the Duke's profile, his profession is Holy Knight, but to have that displayed, he first had to become a Knight level 50 and a Warrior level 30.
So in Duke's mind, Michio is a level 50 explorer and an adventurer level 20 or higher because he finished his quest faster than everyone else to transport the elven goods (He is also twice as young as the Duke, so for him Michio is either a monster or the son of a noble family), and that's also why he entrusted him with the task of buying mirrors and selling amber.
Therefore, if he shows up in front of him or his guards to collect the bounties and he sees explorer 40 on his profile, he's likely to stumble badly.
I did my best to explain