For a few days, I've had a coolant smell in my car. Hadn't had time to get it checked out but since all the gauges were fine and I couldn't find any evidence of a leak in the car or on the ground, I chalked it up to a loose hose or maybe a crack in the reservoir.

Today, it was 70 and sunny and the car felt hot so I turned on the AC. About 10 minuted later, smoke was coming through the vents and the temp gauge was spiked. Luckily I was almost home. Let the car cool off and put in some coolant. Put in an entire jug of prestone pre-mixed. Couldn't tell any difference in the coolant level.

What are the chances it's something simple and not time to get a new car?

 

If it lost that much it would have to be visible.
Possibly lower radiator hose or radiator issue.

Are you asking if any permanent engine damage was done ?
That will be unknown until you get the radiator level back to normal and restart it.

Make, model and year would be helpful in offering a more detailed opinion.

 

2002 Ford Explorer
I'm trying to find likely causes so when I take it someplace, I have a starting point.

 

When you have the smell of coolant, you have a leak. Now where that leak is will take detective work, and you seem to have had a massive or over-time leak from a lower portion of the engine, like Pete said. You have a gallon of coolant in the car, chase it with water to fill it up. I am not sure about the Explorer as to whether or not you have to burp the engine to expel air, but you may have to. Fill up the reservoir half way, too. Then start your detective work. It may only leak once it gets hot and pressurized.

 

Often when you have anti-freeze odor in the cab it because of a heater core leak. The heater core is generally behind the glove box and a leak could be absorbed by the carpet.