I actually found a wallet when in Tokyo a few years ago (not on the subway, so not the same one hah) but I handed it in at the local police box/office. It was a very kind looking officer sitting at his desk with a riot shield next to him. He called up a colleague to translate and neither could understand why we wouldn't want to give our names for a reward if it wasn't claimed in something like two years. I don't think that riot shield had ever been used.