A woman left her wallet, schedule book, drink, and slippers on a bench while she made a call from a phone booth about 10 feet away.
Both natives and foreigners alike are told constantly that Japan is a "safety country", and that people do not engage in petty criminal acts. This is largely true and at least a little false. I left a tote bag in a bike basket while running into a store and it was stolen. People have found and not returned my husband's lost wallets twice. That being said, you generally can leave items out in public in Japan with far less fear that they might be stolen than in similarly populated Western countries. People often leave personal effects in vulnerable positions because they trust that they won't be taken by random strangers, and usually they are not. Just seeing that often gives me hope for humanity, and makes me smile at the nature of people who show this sort of trust of strangers. Also, I have experienced situations where personal effects were not taken. I left my purse behind on a cement wall near a bank the first year I was here, and it was there when I went back for it. My wallet once fell out of my pocket on a park bench, and someone pointed it out to me. A woman even chased me down because she thought I'd forgotten to pack a bunch of broccoli that she thought I'd bought at a shop (though it wasn't mine).
While I absolutely do not recommend leaving valuable items in a situation where they might be stolen (as I have had things stolen for becoming too complacent about leaving them and walking away), you can generally make a mistake and leave something behind with more confidence that it'll be there when you get back.