Causes for discoloration on electrochromic mirrors....
When the wrong type of cleaning products are used on a mirror, it can cause discoloration. It is generally a bad idea to use products
that are not designed for glass and mirrors on these items, because if the cleaner happens to get to the back of the mirror, it can damage
the backing and lead to some of the spotting you see.
Chemical Reactions It is possible that some brown spots on mirrors are due to a chemical reaction that occurs over time when chemical components of the backing and the glass undergo prolonged exposure to harsh sunlight. In other words, the mirror backing may become discolored and cause the spots. A common cause of this can be the adhesive used to hold the backing to the mirror.