Japan is a relatively safe country to leave your personal belongings unattended. People leave their umbrellas in front of the stores when entering the stores. Stores which do not have designated place to keep these umbrellas, they offer special plastic bags especially for umbrellas. Customers put their umbrellas to these bags and take them with them during shopping. These bags keep away the floors to get wet.
May be there is less risk to be stolen, leaving your personal belongings in public areas is not welcomed in Japan due to there is a risk of suspicious items. Therefore do not leave your personal belongings unattended.
Also be aware not to drink a beverage which is left in a vending machine. Most likely these bottles or cans are not left due to the previous buyer forgot to take them but someone had put something inside the bottles.