water damage indicator triggered without it being in water!?

My phone has been acting really strange recently (multiple apps shutting down, long app loading time, extremely bad battery life-- almost 20% decrease with 2 hour use) So I visited apple store today and they told me that all of the symptoms were due to water damage of my phone.

And water damage was least of my suspections. Okay so my phone has been near water, but never in it.

I take my phone to the shower sometimes for music and that's the closest my phone got to water.

The apple store showed me how the supposed to be white sticker inside the charging jack turned red and they told me that my warranty is now void.

They also told me that I need to pay $200 if I want a replacement and that there is nothing they can do when I told them that this phone has never been in water or suffered water related accidents.

I asked them if there was only one water damage indicator in a iphone. The person told me that there were four, two on the outside and two in the inside. He opened up my phone and told me that only one water damage indicator (external one) but they only need one indicator to turn red to void my warranty.

When I repeatedly told him that I never had my iphone near water he told me that it only takes one drop of water to trigger the indicator and that it could be anything like a raindrop. I think this is absurd.

Is there any other ways to prove that my phone didn't suffer water damage? Has this happened to anyone else?