It would be possible for adventurers or experienced knights with a Mana Detection skill to notice him. However, though Vandalieu himself was not aware of this, his Mana was difficult for others to sense due to his aptitude for the death-attribute magic.
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Therefore, a person with a level 1 or 2 Mana Detection skill wouldn’t be able to sense his magic, even if he were to cast a magic right in front of their eyes.

Even with a level 3 or higher skill, it would be impossible without great concentration, but because there were no Devil’s Nests with strong monsters around Evbejia, it’s Adventurers’ Guild’s members were D-class at best, so there were none with a level 3 Mana Detection skill.