Hyundai i30 2013: Smoke from the exhaust

I have a 2013 Hyundai i30 1.8-litre petrol. At about 28,000km, when accelerating in third and changing into fourth at about 60km/h, a cloud of white smoke came from the exhaust. It was so thick I could not see a thing behind me. There was no noise or any other symptom except a giant smoke cloud. After pulling over and checking everything it appeared fine. I went straight to my dealer where it was checked, and I was advised that nothing was wrong. The car drives perfectly, but recently at about 40,000km the same thing happened again. It still drives perfectly, nothing has shown up on subsequent services and the car drives as though nothing happened. What could cause this, and apart from monitoring it, should it be cause for major concern?