Those folks with hunting rifles displayed in the window of their truck weren't exactly smart about it. You wouldn't hang something else of street value in the window of your car separated by a thin layer of glass. Over 8,000 guns are reported stolen each year. Many of them from the glove boxes, center consoles, or under seats of unlocked cars.
When you have more guns than people in this country, they are pretty easily attainable. Unfortunately, continuing to add more than 300K to that number every year isn't exactly fixing that problem like the nra promises.