So - my phone has now suffered a second round of unresponsive touchscreen behavior: scrolling by itself, clicking on stuff at random and generally impossible to use.
The first time this happened, it got rained on at my moscow dacha. So i left it out in the sun for three hours (on the window sill as the sun was coming up, so that isnt an accurate time). I worked fine after that.
And here i am with the same problem in the south of france - it was in a beach bag with a wet towel for half a day and went crazy again. I googled it and found a whole plethora of totally bogus advice from know-it-all weirdos without a clue: its the protector, conductive grease on the screen (complete nonsense), rfi, blah blah blah. None of these are strong enough to cause prolonged ghosting.
But I would have to assume that it is humidity. Remember - the moisture sensor is on the opposite side of the phone from the screen, so it wont show anything you wouldnt guess at after dropping it in the pool anyway. Moisture builds up over the course of hours or days, eventually becoming a blob that actually conducts a significant amount.
Scientific method then should tell. So i put the phone in the freezer for 20 mins. Sure enough - the inside of the screen had three spots of frozen condensation, one the size of a nail head.
I then left it in the sun, screen up, for two hours. Opened up as much as possible - no battery, keyboard slid out. The water evaporated entirely and now i can use my crossword puzzler again. Alternatively you could get a load of books from your bookshelf, and put them in a airtight space (large drawer/ storage bin) with the handset, but that takes longer. You can dry many things this way...
Also, to anyone who wants to discourage using the sun - dont be alarmed! Its one of the best ways of drying stuff because of the contact with open air and the relatively low temperature.
Oh, and it might start working after a half hour or so, but its best to give it a good soak in the heat as a quick dip may only disperse the moisture around the inside, leaving it to condense again later!
Hope this helps... and remember too much sun and or freezing may be bad for your set