Thank you for the comments! The white triangle is actually the opposite window of the car. I was in the backseat when I took this picture so the mirror was actually reflecting the sky behind us as well as the person in the front passenger seat, the driver, and the driver's window. I snapped off a shot and then realized that more than just the sky would be in the picture, so I used a red filter and suprisingly it got rid of all but that triangle. I would have taken it out in photoshop, but I wasn't sure if it was either too distracting or an interesting point in the shot.