Have the phone in a really cold air conditioned place recently, then walked out into a hot day (or into a hot car)? The moisture in the hot air will condense on the cold phone. (I've seen it happen badly enough that the phone was water damaged.)
That's legally considered normal usage (going from a cold place to a hot place), so you can probably get the phone repaired (or replaced) under warranty - if you're willing to put up enough of a fight (which might mean a law suit).
A US soldier in Viet Nam had a pot (the kind you cook in) replaced because it was guaranteed to stand up to normal use. And the courts held that, in Viet Nam, during the war, bullets flying around your cook fire was normal use, and there was no "act of war" exclusion in the warranty.