It may have simply fallen out. Or, it could have been swiped by airport baggage handlers, or TSA. TSA is supposed to leave a note in the bag if they check it, and usually they will mark on the outside luggage tag also. (They are under no obligation to re-pack your bag they way you had it.) TSA bag search areas usually have video surveillance, which definitely discourages theft. But it's possible. Baggage handlers are the more likely culprit, if it's a case of theft. They generally make less money, are vetted more poorly, and have greater access to the bag, especially in areas such as cargo holds where there is no video surveillance. Nonetheless, I'd contact TSA (soon) and ask them to investigate. They do look into such matters, and when they find theft they can (and do) fire TSA personnel. I can't really advise you on the airline aspect because I know less about their operations, but it can't hurt to file a claim.
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