Check your coolant cap. It should end with a .02 or .03 if it's one of the updated models. If it was pressure tested, that should rule out the entire system including lines, hoses, radiators and pump. Generally a pressure test device screws on in place of your coolant cap, so this only leaves the cap to be a failure point.
Being that you lost almost a gallon in a few miles, I would imagine that you'd see some water in the trunk, or condensation in the lid, if not damp carpet in the trunk. If the cap leaks, it usually dissapates as steam leaving a bit of mist on your trunk lid rather than a full stream of leakage. Another tell-tale would be the odor of coolant when you open your trunk.
Should you need a new cap, check with Suncoast
www.e-partssales.com they have pretty low prices on OEM parts. Good luck!
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