But Apple would refuse repairs based upon one of the exposed sensors turning pink. After the initial flap, they started opening the case to see the internal sensor, and that only if both external sensors were tripped.
Former Genius here.
When I was in training, in Cupertino, my trainer was talking about these sensors and what exactly it takes to set one off. I challenged him to demonstrate. So we went and got a 3GS, opened it up, and I took a toothpick and put a droplet of water on the sensor. Like literally had a drop of water, held by surface tension, sitting on the sensor dot. And it still took a solid 20 or 30 seconds to turn red. That was absolutely the best thing I saw in training (well, that and I got to see Steve Jobs talking to Jony Ive from a distance in the cafeteria, but that wasn't strictly relevant to my training
)
Now, I'm not saying I don't believe 3M when they admit that humidity could have the same effect. They made the sensors; they would know. But the sensors don't just trip from casual, momentary contact; they do require the sustained presence of water before they'll turn color.
At the Genius Bar I saw people who had been, for example, out in the rain and unlucky enough to have a raindrop go right down their headphone jack and trip the sensor. I also saw teenagers who would leave their phone on the counter playing music while they took a long steamy shower and got the sensor tripped that way. My personal "favorite" was an older woman who had put her phone on the nightstand next to a glass of cold water, and in the night, the condensation on the glass built up, ran down onto the nightstand and into the 30-pin connector. Bad luck.
As a Genius you're
supposed to make a judgment call about when to make an exception to the rules (or at least, when I was there you were supposed to; this was back in the Ron Johnson days and my friends who still work there tell me that under Browett there's been much less flexibility). So I would try to still help these folks out when I could.
Anyway, just wanted to make the point that, though there are a lot of bad-luck ways these sensors can be tripped without the owner necessarily being careless, they don't trip for nothing. They really do take some serious moisture to be set off.