>>5: excellent description, thanks a lot.

I have read about this, and it seems that it comes from a faulty integration from personal sensory information. In fact your brain ils placing your body wrongly in your personal mental space, and basically the person behind you is yourself. For some reason the misplacement seems to be always on the back, but I'm not too sure about that.

In any case, these types of illusions can be induced artificially by electrically stimulating the brain.

Maybe someone hugging you helped because that extra-sensory information helped your brain to correctly place your body in your personal mental space.