In two weeks that person hasn't communicated to Turkish. It was hand luggage and it took absolutely no time to see, before boarding his plane, that it was not his handbag. It is something too obvious. The one my father saw had NOTHING similar to his. It was some sports bag, while my father was a small backpack. So there is already a difference.
Not having reported it in now 3 weeks, means he is confortable keeping my fathers belonging in his final destination, and his own belongings were not so precious (few dirty clothes, and some brochures in Arabic or Farsi, that was all).
I believe both the airline and TAV should have done something, like check the CCTV and track the person who then mistakenly took the bag (now mistakenly stole the bag), ask him to open it, and realize it was not his. My fathers driver's license or credit cards there would have corroborated his identity, therefore he would have received his bag instantly, and TAV/THY would have gave the sports bag to that passenger, assuming it belonged to him.
Contact details of my father were almost on every place of his wallet and his phone. He had his personal cards, with email, mail address, even a fax number (who uses a fax today!), plus ID and Drivers License (in Argentina ID is called DNI).

Maybe security control in transit is not needed. I haven't seen those in the US, and they are the ones who defines the rules concerning security.