I do not think it is irresponsible to leave a rifle or handgun in a car while eating a meal, enjoying a swim, going horseback riding, or any number of activities that preclude the carry of a gun. Sometimes, there is no other choice than to store a firearm in a car.
I freely admit it is more likely to get stolen, but sometimes that is the lesser of two evils. What do you guys do that work at a place where you cannot carry into the building? Do you go unarmed both before and after work, or do you leave it in the car? Just curious, because your car is probably parked at work for more hours of the day than it is parked at home.