Yeah, it hapened to me a couple of days ago. I was driving along, and the road was wet due to a leaking mains pipe. Nobody in front or coming the other way, but as soon as I hit the wet road the wipers went. As far as I'm aware, no water hit the screen. Was it a coincidence?
I find the wipers wipe when driving in misty rain too often. In normal rain it is great, though.
The sonsor is an IR thingy that sends out a beam of light at an agle to the screen, and measure the amount of IR light reflected to work out how much water is on the screen. Could a bug or some other object hit the sensor and confused it?
I've had the same situation. I put it down to the sun's rays fooling the sensor into thinking that it is raining. I don't leave mine on Auto for that reason. No doubt the wipers will come into their own when autumn/winter is upon us.
However the Auto lights are great for going into tunnels and car parks.
The worst thing with the autowipers I found when I had the Golf and the Laguna was that on those trips when the washer bottle runs out, if you forget and leave the wipers on auto they do agreat job of reducing vision to almost zero when you pass a truck in a mild shower and the crud on the screen gets spread all over the show.