Added this to section 2.1.3 of the Codex:

[quote]The sixth sense forms an image of the soul’s surroundings by sending out waves of [b]scanning[/b] magic, a process not dissimilar to echolocation. It becomes weaker the further away it expands from the origin, and will have more trouble penetrating dense, powerful materials; the user will then need more power or skill to make the scanning more effective. The mana of this magic is usually faint, and requires a notable amount of skill to detect, but more powerful scanning magic is naturally easier to detect. Additional magical effects can also be used to make the scanning more difficult to detect.[/quote]

Again, those are things that have always been true, but I just forgot to put them in the Codex. I'm too accustomed to my own setting, and tend to subconsciously assume that other people are accustomed to it too.