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Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern but most likely, it's normal.
It's certainly possible that he's expelling built up heat thorough panting since dogs don't sweat like we do, except through their pads and nose. However, dogs can also pant when they're excited or anxious. So, he may just be happy to be going home with you as well as being excited about a car ride or he was playing before you picked him up and he is, indeed, winding down as you suggested.
I wouldn't worry too much about it as long as he doesn't get tired easily and there's no evidence of respiratory distress at other times.
I hope this helps to reassure you about him. Deb