mine does that if i haven't driven it in a couple weeks, i think its because its pissed at me for letting it sit too long.....
Make sure the carpet below the dashboard is DRY. This might be an indication that moisture is collecting on the floor or behind the dash somewhere...
I had the same the thing happen many years ago to my car one night, many years. At a show, the day was sunny & warm but at night it got cols & humid. Lenses fogged inside. With the heater on, the fogging disappeared and never happened again.