In this case it wasn't his magus status that allowed his detection, but rather a very specific gimmick.
One with no explanation and just a title. Oh, and the way he got it.
It doesn't beg clarification, it demands it. We can't have stuff like this appear out of nowhere when it's convenient more than it already has, because the last few times(Raul sneak attack, for example) there were justified calls of "bullshit", and nobody likes a shitstorm.