Another possibility is the cap on the coolant overflow tank

is not sealing well. They are prone to cracking. The leak

might not be noticiable until the car warms up.

There is a newer green/teal/blue (depends on who is looking

at it) that replaces the older grey ones and is less likely

to break. I understand you need to be careful to not

overtighten the caps.

Also, would aleaky heater core cause the interior to fog up?

I've never had that problem but I would think that could

happen.