cooling system

Which engine size? At what speeds does this occur? What are the symptoms or observations?

This is my wife's car we are talking about. I did not see any leaks after filling the radiator with water. She said she started smelling something and she noticed the indicator at H. She just pulled over and tried calling me (fourteen times) she said. Meanwhile, she called AAA and got a tow to the house. Anyway, after checking for leaks and finding only a small very slow drip, I took it out and the gauge never went higher than half. I am going to add coolant this morning and check the other fluids. I am keeping my fingers crossed that this was just a matter of not checking levels. Thank you for your response!

If the coolant was low, it had to go somewhere. If you saw a drip, that is where you need to look further. External leaks are usually the least expensive to repair. A pressure test may make the leak easier to find. You might be able to borrow a tester from an auto parts store that rents or borrows tools. If the leak is too slow to find that way, you can add a small bottle of dark purple dye to the coolant, then check a day or two later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. Auto parts stores will have the dye, and they should have a black light too.