Occasionally I used to leave a bass, in a black hard case, after dark, across the back seats of my car (which are black), in a supermarket car park. After practice, never after a gig. Still a risk, but quite small.
The problem with leaving anything evidently nickable in your car is that even if you're not out any cash, you still have to get the car repaired. And I wouldn't want to explain why it was broken into to the insurance company.