I got a second hand U1 today. I noted that there are serveral very tidy dust like white dots on the surface of the crystal. I cannot remove them with camera lens cleaning cloth. It looks like they are beneath the surface of the crystal. I don't understand how this happened. The watch is supposed to be water proof. Does it mean the watch has been opened or is there any other explanation? Is there any way to deal with it? Anybody has similar experience?