Well, accidentally or not, he took it and never gave it back. Both TAV and Turkish Airlines have checked multiple times in the lost objects area and nothing has been found. NOTHING. It's been now three weeks and the person who, by mistake took it, forgot to return it back. So it means he kept someone else's private property, and that is a definition of theft, without violence.
Last year, I boarded an Aegean crowded A320 for the route JMK-ATH and I found, while in flight, an IPAD there. I reported it to the crew, she took note of the place it was. As soon as we landed, she gave to Aegean people, who contacted the passenger who had forgotten it. Then, the IPAD flew back to JMK and there was this person waiting for it in the airport. Easy, very easy, and speaks great about Aegean, but Turkish, well.... There is a difference!