In cold weather it does not take a leak to get water inside the passenger compartment. Human breath, liquid water/snow tracked into the vehicle on your boots, a car wash, light snow drawn in through the inlets, etc. can easily put enough humidity into the air so that when the vehicle cools down overnight condensation can form on the inside of the windshield.
Just use the defroster and make sure you do NOT have recirculation 'on' (you want to get fresh outside air in to lower the interior humidity). Chances are this occurrence will be rare.
If a true leak exists, then of course you need to see the dealer pronto.
Also, I've never heard of this idea of turning off the heat before shutting down the engine. My cars have always had the heater left 'on' from about October to April for decades now, running or not. The difference in engine warmup is negligible in my experience, and certainly not detrimental, compared to human comfort. But then maybe this is just a Great White North thing?