If it was a heater core leak, you would smell coolant. Turn the heater to max hot with fan on low and see if you notice any steam or coolant smell. Also, how is your coolant level?
I bet if you soak up some of this moisture with a white paper towel, aside from dirt you will find that it is just plain old water, probably from a windshield leak. You did state that the windshield was disturbed a while ago?
Occam's Razor