I think what it boils down to is taking a calculated risk. I wasn't able to carry at my last job, so I left my weapon in my car... Because of where I worked and the fact that there was secured parking with 15' fences I didn't even use a lock box, I just slid it under my seat in my paddle holster. That was a calculated risk, based on the factors that I had and what I knew about the area. When I do leave my gun in the car elsewhere I will lock it in a car safe with a cable, or locked in the glove box or even locked in the safe in the glove box. I rarely ever go somewhere I can't carry just based on principal, or I will just cover it up... concealed means no one knows right? The last thing I will do is just not take a weapon with me, which is rare. But if I feel there is enough of a risk of theft, and I don't like that risk then what take it?
I think that the solution to the question is far more complex and I think that the OP needs to looks at the situation, where he intends on parking his car, the people that might be around, what kind of car he has, if he can check on his car throughout the day/night, is there an alarm, are there other people around, etc etc...