What does "theft of a Bitcoin" mean?

The British legal definition of theft is defined in the Theft Act of 1968 as follows: "A person is guilty of theft if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it"

Just because Bitcoins aren't tangible, doesn't mean they aren't "real" in some sense - they are as real as a bank balance number. And stealing applies to illegally transferring money from one bank account to another, even if the transfer is data rather than actual bank notes. So the definition applies.

Obviously hacking into someone's computer, taking their private keys, and using them to transfer Bitcoins into your own account is not just theft, it involves the crime of breaking into someone's computer too.

But what happens in the very unlikely event of guessing someone's private key. Well, apparently finding and keeping something that isn't yours can also constitute theft. It's called, unsurprisingly, "Theft by finding", and is a criminal act. So if you manage to guess a password you have to go to reasonable lengths to find and inform the actual rightful owner of the Bitcoins. Or just leave them there.

Of course in practice it could be argued that if a simple web search doesn't associate the public key with an identifiable person you've done your bit. And in any case, siphoning out the Bitcoins followed by laundering them through a Bitcoin mixer service would make tracing a thief almost impossible.

There really is a market for a reliable and responsible service to secure your Bitcoins. Something like a traditional bank, perhaps?