Setessan… Something

You perform a mid-round deck check on Alex during round 6 of a Theros sealed 5k, of which you are head judge. You quickly discover that he is playing a in his deck, but no copies of this card are marked in the Total column on his deck registration sheet. When you investigate further, you find that a is marked in his Total column, but it is not in his pool. When you speak to Alex, he says he didn't notice the error because the card wasn't in his main deck, so he didn't bother to check that it was