What are the steps to troubleshoot a coolant loss from radiator without drawing more from the reservoir?

What is your question?

There is no water in oil, low side line is not collapsed, and heat/ac operate as they should. I'm stumped. No noticeable leaks. Only loses coolant at extended idle...new radiator cap

What makes this problem better or worse?

exaggerated with 90 degree weather

How long has your 1995 Nissan Pickup had this problem?

3 weeks