Body sweat can be an issue as well, depending on where on your person you keep the phone. I had this issue with a Razr. It quit working and the little telltale white dot had turned red and I had NO idea how or when it could have happened. When the phone was returned unrepaired because of water damage, I disseected it all the way and I could see where moisture had indeed gotten inside. At the time, it was summer and I was keeping the phone in my front pocket. Bad move.
If they saw water damage, then it came from somewhere, without your knowledge or otherwise. Something happened. The phone makers have much better things to do thatn to piss off customers with false claims of water damage.
Sorry, wish there was better news for you.