Gosh looks like someone has the same issue as me! Recently, I bought a Cartier Santos 100 for my other half. Then one fine day, she told me that certain part of the front glass like 'blur'. I took a closer look, it doesn't look like scratches. I cleaned that 'blur' portion by rubbing it with those soft watch cloth. And magically the 'blur' part disappears! But after two days, the same problem came back again and exactly at the same spot! This really puzzled me. And same thing, I cleaned/rubbed that area with the cloth and all is ok again. This goes on and off and for a few times over the past two weeks. And I think the rubbing action generates some heat thus the 'vapor' disappears...and so 'blur' part also gone...
I begin to think hard and think it could be due to some tiny vapor being trapped between two pieces of glass being sandwiched together. So sort of just like the case over here. Oh man what should I do?