As a retired cop and having had to take or review thousands of theft reports during my 30 year career, locking your bike in a car is better than nothing, but not as secure as a bike secured by good locks/cables mounted externally. “Smash and grab” thefts are far more common due to the ease of breaking a window, opening the car and splitting - it takes much more effort to defeat a good, metal lock/cable. Windows weren’t designed as a security device, it’s merely a secondary function. A couple ways to improve locking items in your vehicle is to keep them out of sight, dark window tinting or parking your car between two others so they can’t see it or extract it as easily will create a mild deterrent. It’s amazing how fast and quiet you can remove a car window with a spring-loaded nail punch and shelving paper.