Probable cause and CCW with a permit
Licensed carrier carries concealed on Atlanta's MARTA mass transit, is detained for half an hour and released. A federal judge rules the detention was lawfully based on probable cause.
Here's the rub: if the offense of CCW were worded as "it is unlawful to carry a weapon concealed without a permit," then lack of a permit is part of the offense, and until there is reason to suspect that, there is no probable cause. But if it is worded as "It is unlawful to carry a weapon concealed. Exception for people having permits," then concealed carrying is the offense, and sufficient suspicion of that justifies an arrest or detention. Having the permit is a defense, and the officer doesn't have to rule that out, any more than he has to rule out insanity, justification, etc..
I think a separate question here is the length of the detention. Absent true arrest, the detention must be reasonable, and no more than is required for the proper purpose. Here, once the person shows his permit (and perhaps the officer verifies it is valid), the detention should end. I doubt suit would have been filed if the officer had asked for a permit, he'd shown it and matter had ended there.
Hat tip to readers Ryan Gill, Scott Nolde, and Ed Stone...