I always thought it was a defroster because that area of my window starts to defog when I put on the rear defroster. Could be the heat from my rear window, though, that is making that area clear up as well. I believe mine covers several square inches of the window, though.
I only noticed it was defogging in that area when I did a head check before changing lanes. If it is a defroster, that could be the reason why it is only on the passenger side.