From your link:

  Among U.S. states, Alaska, Arizona, Arkansas, Vermont 
  and Wyoming allow residents to carry a concealed firearm 
  without a permit. These states also allow the open carry 
  of a handgun without a permit.

I count five.

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.

Several states have permissive open carry: http://en.wikipedia.org/wiki/File:OpenCarryUnited_States.png

Even if they were concealed carrying it without a license, the statistic would be "10% of those caught not in Vermont did not have a license to carry the gun in the manner that they were carrying it".