At the pet supply shop we have reminder signs posted about leaving pets in a hot car.
One time my husband and I went to eat with my folks and we came back and there was a young baby in a baby seat in back seat of the car parked next to us...car locked..windows up...my dad, who worked security before he retired, noticed it first actually and became very upset and wanted to do something, he was considering breaking in, calling police etc...as we all peered in, I realized that it was VERY lifelike- baby doll strapped in a similarly lifelike baby seat - NEXT to an actual <for use> baby seat. In the back.
Once I was at the a store near the shelter where I volunteer. People at that store know that I volunteer at the shelter. Someone came to get me because they were concerned about a small dog in the back seat of a parked car. They wanted to know what should we do? We looked in and I noticed that all the windows were cracked and the car was parked in the shade; dog did not appear in distress and the day was not "hot and summery", but actually a cool Spring day.
Considering all this, I thought that the dog was fine where he was. But that was mostly because the temperature was not at all seasonal for the area and all windows were open some.
That said, in terms of temperature I personally wouldn't leave my dog in a parked car during anything other than Winter. Someone would be very stupid to try and mess with my car with one of my girls in it but for behavioral as well as safety reasons I generally wouldn't do it.
There's been a couple of times I stopped for gas on the way back from the vet's because I was very low, and I had to use the bathroom so I literally ran in and out.