Not sure how the BIP39 passphrase protected mnemonic code would not reveal your balance. Care to explain this? Thank you.
It works by basically deriving two different seeds:
Mnemonic Code -> Seed1
Mnemonic Code + passphrase -> Seed2
Seed1 is here used for small amounts. Enough funds to let the thief be happy, while only being a small portion of your total wealth.
Seed2 derives other private keys. These are used to store the majority of your wealth.
It basically allows you to have a "hidden wallet".
Without the passphrase, there is no way to know whether there is such a "hidden wallet".
Any passphrase can be used to derive new seeds. But only the
correct one will derive the seed which is being used to derive the private keys for holding the majority of funds.
Adding to what bob123 has said. @zoltanb, with the same mnemonic but different passphrases you actually may have as many "hidden wallets" as you want. Something like this:
This approach to hide some sets of private keys is widely used by owners of HW like Ledger and such.