The iPhone moisture indicator in the headphone jack may not be as accurate as Apple say when it comes to moisture damage. Apple specs say that it should survive through a temperature range of 20c to 45c; however apparently cold temps could well trigger the iPhone moisture indicator.
Moje Jabluszko, a Polish website conducted several tests on iPhone handset to determine if winter conditions would affect the iPhone, especially the iPhone moisture indicator and if it would turn red with temperature change.
Of course we have that iPhone test video for your viewing pleasure below courtesy of the guys over at gizmodo, and although the tests may well not be without flaws there is a suggestion that the iPhone liquid sensors are a tad inaccurate.
The tests show that there is a possibility that the iPhone liquid sensors may turn red at 11c rather than the Apple stated 20c, and although it is purely scientific testing, the iPhone liquid sensor is supposed to be an indication of water damage and spills and not a change in the weather.