Back in the day when I was a young police officer, I would occasionally take my off-duty gun with me when I traveled on business (my off-duty sidejob was government relations consulting).

Off-duty gun had to be in a checked HARD-SIDED piece of luggage, and had to be inspected by the ticketing agent before being checked to make sure it was unloaded. Bag was then red-tagged.

Always thought that the red tag was a screaming signal to STEAL THIS BAG