On one of my other cars, when I had a Heater Core leak, it showed up as fogging my interior windows, after a shut down, and the car sat for a period of time.

It wasn't pretty, and for sure it was very unhealthy to have those fumes circulating in the cabin (although there is a little better airflow in the Roadster)...lol...

The heater core did eventually affect the coolant level, but it was very subtle and not easy to detect via a level check.

Might be wise to visually inspect the Core, cause if that's the issue, you don't wanna be breathing that $hit in for an extended period...