Bag, laptop, wallet, cell phone... As I know, most people just leave it at the table or take it with them if it could carry enough. However, if you are not feeling safe, you may entrust it at the counter and go to the bathroom.
I always leave my belongings unattended at a restaurant and a cafe. Nothing has happened. Korean people are not interested in other people's personal belongings. But if you're worried, it's okay to carry your bag with you.
Sometimes even if there is CCTV, it is difficult to catch the thief. you might need to be careful same as in your country. rather, Koreans tended to be Too lean on CCTV, and too careless about their own belongings than foreigners.🤔
I leave everything behind.. even my cellphone on the table to go restroom. I haven't felt unsafe so far. It is not about CCTV... it is about kinda everyone is watching everyone. It is like implicit agreement between people in such places.