No issues in mine, have been using it for more than 2 years. Water and humidity is not the problem, mine is subjected to those quite often. Has it come into contact with any corrosive atmoshpere? It probably wouldn't be covered under warranty.
Definitely didn't subject it to any corrosive environments. I've been using it more than a year and this discolouration wasn't gradual, but happened overnight while the watch was just sitting on my desk in an air-conditioned room. It's quite weird that I couldn't find any clause for discolouration in the warranty terms too.
It has been sitting on the same desk since quite a long time. I don't think there's any possibility of any induced corrosion. I will try cleaning it with some alcohol and see if it makes any difference