Ah—my misunderstanding/miscommunication. In Alaska, for example, it wouldn't be against the law to normally carry a firearm, and thus it would be entirely believable that someone left a firearm in their backpack. In California, it wouldn't be: if the guy doesn't have a concealed handgun license, he wasn't able to legally carry the gun outside of his house, much less into the airport. The "I got careless" argument makes way less sense.