mechanic most likely didn't bleed system properly/fully...
Total coolant/water capacity, depending on MY/options, is 5-6 gallons so 1/2 qt is not much. If you were to put 1 gallon in, I'd be worried... among other things about the mechanic's capabilities.
Keep an eye on it and/or do a bleed yourself (search and you will find complete details).
Good luck
