Bitcoin Billionaires, Ben Mizrahi's book on the Winklevoss twins after their settlement with Zuckerberg and Facebook, has a chapter on how they flew around America each depositing parts of their keys in seemingly random and unconnected small branches of different banks in safety deposit boxes for the bitcoin wallets that they were storing 10's of millions of USD worth of Bitcoin at that time.
It seemed like overkill then... they were spending millions at $7 to $10 per BTC.
Here in Israel, way too many government services can be reached by knowing your ID number and your mobile phone number. If you have access to a cloned mobile number (and I'm pretty sure this isn't that hard to do here) you're in trouble.