Cafés and restaurants may be crowded and tables scarce. You need to secure a table before buying a tray of food and drink (else you could be standing around for a while til someone leaves).
A common sight – these people have left a handbag, suit-jacket, a phone, and a briefcase to ‘reserve’ their tables while they have gone downstairs to probably stand in a queue to buy more coffee or snacks.
Japan has an extremely low crime rate. Their personal belongings are safe.
Japan Tourist Tips 