Technically, your back is facing away from your front. So for someone to "talk behind your back" they would be in front of you. Your back's back is your front, amirite?
But from your perspective, anything behind your line of sight is "behind" you. So talking behind your back would actually mean behind your back, not in front of you.
But from your perspective, anything behind your line of sight is "behind" you. So talking behind your back would actually mean behind your back, not in front of you.
Almost word for word what I was going to say.