Recently a friend of mine dropped his (waterproof) phone in a river, and we had a very hard time to find it again. So, we thought about what could be improved to make spotting the dropped phone more easy. That's what we came up with: The phone could sense that it is dropped in water, and start flashing the camera flashlight on the backside and the screen on the front. In this way, the phone would stand out against a dark or muddy background.
The question only is: can the phone somehow sense it? I think it should be possible because capacitive touchscreens do not work under water, so apparently the presence of water on the screen can be "understood" by reading the touchscreen sensors, right? Then it would only be a matter of software!