For those in the US, a 70 degree F car on a sunny day can reach temperatures over 100 degrees in minutes. While telling people to never leave dogs in cars is extreme, the margin for error is quite small once the temperatures climb towards 65+ degrees, and if you can’t be absolutely assured that your car will stay under 65 F, then don’t take the chance. Leaving the windows down does not matter.
If you see a dog in a hot car, call 911. Or get a brick.