RD,
I recently had a similar problem. I thought initially and hoped that it was just water vapor but it started happening more frequently on non-humid days and had that musty glyco smell. I also topped off my coolant because it was a bit low (it never had been low before)about the time the vapors started. Turned out to be leaking heater core as diagnosed elsewhere/eariler on this board.

I used Prestone extended life (orange colored stuff) in my car, maybe other have better recommendations.