Quite simply they can ask you to prove the origin of the bitcoins when you eventually want to use them/convert them to fiat/etc. Didn't buy them on an exchange

Quite simply they can ask you to prove the origin of the bitcoins when you eventually want to use them/convert them to fiat/etc. Didn't buy them on an exchange or from another person? Well, then you probably mined them and they'll be taxed as income.