Hi and thanks for using 2CarPros.
Since there are no leaks inside or issues with the windshield fogging, it sounds like the heater core is fine. So, that leaves everything under the hood to check. I am going to supply a few links that discuss finding and repairing a coolant leak. Follow them and see if you can locate where it is coming from. Also, you will need to pressurize the system. Most parts stores will lend or rent the tool to you.
https://www.2carpros.com/articles/coolant-leak-symptoms
https://www.2carpros.com/articles/car-is-leaking-coolant
https://www.2carpros.com/articles/radiator-pressure-test
Let me know if this helps or if you have other questions. Also, pay attention to see if there is any evidence of coolant under the vehicle after sitting. If you place a clean piece of cardboard under the front, it will help locate the leak.
Let me know what you find.
Take care,
Joe
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Monday, February 18th, 2019 AT 5:50 PM