Do my opponents get a penalty to see me if I sneak up from behind?

My friends and I started a Dragon Age Roleplay and in the situation “A rogue approaches a creature from behind,” we must test the creature’s sight against the rogue’s dexterity. An argument arose that the creature doesn’t receive a penalty for facing the other direction. The GM said that it’s not written in the rules, while I think that it’s only natural for a penalty in the opposing check.

Can someone help me with that argument? Does the creature get a penalty, or not?