Xzight, not sure I understand your question. Are you asking for a rule reference that would tell you that a wall does NOT offer protection to a squad when it is NOT between the attacker and the target? Beside the obvious, some of the module TEC do say that walls give a +1 TM against direct fire traced through Wall hexside. They also say that Hedge negates movement modifier against direct fire traced across Hedge hexside. Both are then followed by a sentence mentionning that they don't offer protection against indirect fire (which lobs shell over obstacles), implicating that there are some directional aspect involved.
More indirectly, in the LOS section of the rules AND the TEC, there are plenty of examples mentionning that hexside terrain will affect LOS only if said LOS cross the hexside terrain. It make sense that a wall does NOT affect LOS if it is NOT located between the firer and the target right? So it makes sense that the same goes for fire.
Now, if you are looking for the -2 rallying bonus hexside terrain can give you, it is on page 31: All units in terrain with a positive Target Modifi er (+TM), including those with hexside terrain like Walls (if any/all enemy fire would cross the Wall hexside), subtract two (–2) from their die-roll.