Basically, information(e.g. passbook) left by decedent should be the footprints to trace.
There is no system to seek for the bank account with common IT network among banks. We should make inquiry to EACH bank for existance with bank account together with balance. Each inquiry requires the proofs as heirs and proof of death. Online requisition is not available – all must be done at bank branch counter with showing originals of proofs. Note that such inquiry will freeze the bank account, immidiately. Due process for dividing inherited assets should be required for requidation. There is no exception.
Heirs are recommended to seek for the remainings left by decedents, carefully in order to trace bank accounts. Passbooks are apparent footprints. Other communications by postal mail from security company will be leveraged as well.
Recently, however, internet bank are more common. Such banks will not issue any passbook. Even traditional banks are shifting for non-issue of passbook being concious of operating cost.
Heirs are also recommended to look into PCs which decedent used and look at bookmarks. Bookmark might show the banks – which is best trace for bank account.
Above sounds very conventional and manual to you – but as a matter of fact, this is the case to look for the banks in Japan anyway.