Callen, you make a good point here, but I'm not 100% convinced. I think this is referring to spells such as Veil of Invisibility, right? My interpretation is that it is the magic of the spell that thwarts SS, as the two magical abilities compete.
But I also think that faced with an illusory wall, SS could let you tell it was an illusion, but I don't think it could let you see what was behind it. So I think SS must channel through one of your existing senses to give you information.
This may be one of those cases where every picture of how it works is contradicted somewhere...