If you don't have airconditioning, my whole theory gets shot down.
Let me ask something else:
Has it continued to happen since that one time?
Does it happen every time you turn your car off?
Do you remember running over a puddle that day right before parking the car (was it raining)? This last question might seem silly, but if you run over a puddle you will get the undercarriage wet and the water will begin to drip onto hot components (ie. muffler, catalytic converter, etc.)
When you do find out what it is, let us know. I am kind of intrigued!!
Sorry I couldn't be of more help.