This does not have anything to do with aggression.
It depends on how hot it is outside, if the sun is shining on the car, etc. Is it hot when you get back in the car?
I would agree that heat is usually more concern than theft. Why do you have to leave the dog in the car at all? Can you stay out there with him? One thing you may tell your dog is that it is more frequent for "animal activists" to see a dog in a car with rolled up windows and break out the windows to "save the dog" even if the conditions don't warrant it.
NAPWDA Certified Cadaver Dog
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=GSDs) (hemangiosarcoma=blue):Grim , Cyra, Toby
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, Arlo & Waggles