[anigbrowl]

Most legal activity (though not necessarily all) will involve paperwork and receipts, not least for the purposes of documenting your taxable income.

Civil forfeiture is often (though not always) used to clear up the legal status of assets found abandoned during a criminal investigation, eg if the DEA interrupts a drug bust and the buyers flee, leaving a few hundred thousand in cash behind. You could turn up to such a hearing and assert a legal claim to such assets along with an explanation of why they should not be forfeit, but if the assets are the proceeds of criminal activity then it's not worth it unless you have a truly outstanding cover story because you'd risk arrest or having the testimony at the hearing turn up in your criminal case.

IIRC in this case the feds got the bitcoin wallet information by seizing the Silk Road server, so they have a wholly reasonable presumption that the bitcoins are the assets of the (obviously illegal) drug brokerage business and probably a transaction log to back up this presumption.