Well, with a mirror several things can be taking place. The fingerprints and dirt obviously are one, and we can clearly see that happening in this photo. But those are fairly easy to determine. But there are other "possible" things which could cause mirrored effects.
First there are two types of mirrors. First surface mirrors (usually found in scientific grade devices such as microscopes, telescopes, and laser optics) and second surface mirrors. The difference is which surface of the glass is coated with the mirrored coating.
First surface mirrors have the very outer edge of the mirror coated. This means the light does not pass through the glass to be reflected. It reflects directly off of the outer edge. These are usually much more accurate and not as prone to visual effects.
Most consumer grade mirrors however are not this type. Instead the back surface of the mirror is coated. Which means that light must first travel through the glass, reflect off of the mirrored surface and then pass through the glass again. Because of this you can actually get multiple depth effects of smudges on the glass. Also you an get effects if the mirrored coating is scratched, weakened or separating from the glass. Because of that you can get strange visual effects such as this.
Not saying thats whats happening here, but that is a possibility. The only way to know for sure is with multiple images of the mirror from different vantage points, or video which could move through several linear vantage points in its motion.
