The spellcasting section specifies (emphasis mine):
Area Spells: Area spells can be cast on a specific target or a point in space that you can see. Unless noted in a spell, the area of the spell is a sphere centered on the target with a radius in meters equal to the Force of the spell. All targets in the area of effect that you can see, friend and foe alike, are valid targets for the spell. If a potential target is outside your vision (behind a screen, for example), they’re not affected.
I don't think it make sense that a fireball would just not explode into places you can't see, especially considering the note on indirect spells:
So you don’t really need to be able to see the target—you can cast these spells blindfolded or with artificial image enhancement— as long as you’ve got a clear line of fire. [...]
So I was wondering if it only applies to direct damage and effects like illusions and such, and indirect spells like fireballs would affect all targets in its area regardless of whether the caster can see them or not?