Surely this is a simple "duty of care" case. The TSA has a responsibility to ensure that all items it asks to be passed through security checks are returned to the correct person.

It doesn't matter if it was a TSA, other airport or random person that stole it, the simple fact that the TSA asked for it and failed to ensure tha lawful owner received it back it is enough to prove the case.

Though no doubt "National Security" will be invoked to muddy the waters