I'm not 100% certain, but I think it would be something like the little thing in the battery compartment of a cell phone that turns red if it gets wet. That's how the phone companies know if something is water-damaged, because water damage negates a warranty.

Cell phone companies put a water damage sticker inside of the back of the cell phone. This sticker is placed near the battery, and will turn pink or red when the cell phone has water damage. If the cell phone becomes damaged, the cell phone company will open the phone and check out the water damage sticker. If it is pink or red, the company does not have to honor their warranty unless it specifically covers water damage.

We have had the inconvenience of cell phone water damage several times. A few times, the phone was accidentally dropped in water, and one time it fell in a glass of coke! One time, however, the cell phone was placed in the pocket of a damp swimsuit, and it was still water damaged. So you really do have to be careful with the phones. I have learned to have my kids put their cell phones in plastic ziploc bags when they go anywhere near water!

You can now purchase warranties that cover water damage as well. However, I have found that they are ultra expensive, and not worth the price unless you have a very high end phone.

Good luck!