I remember reading about this, not sure where…
Many years ago swiss banks started up the old deposit box thing because countries stopped their residents from having swiss bank accounts, you could get a note of authority and a key to a safety deposit box, the note granting you sole access. Whoever had the note and the key (you could have more than one note/key) could access the box…it wasn’t a registered account. That exonerated the banks from responsibility and also ensured anonymity, but obviously it had its flaws – like being murdered for the key! They eventually had to stop that too I think (not sure why) but those people already with SDBs were honoured the agreements. These were often then sold for good prices on the black market.
There you go a bit of history that I can’t vouch for 
The mattress is probably a better option these days.