Large amounts of cash, and not even all that large ($10k) have been considered evidence of crime and confiscated by the cops in the US. The laws that enable this have been slowly dying off, but it still happens. No other evidence of a crime is necessary nor do the people need to be convicted of anything, the cops just take the money because it might be used to commit a crime.
They check cargo holds and do random bag checks - as of the mid 2000s due to the "war on terrah" but maybe that went out of fashion in the past decade. Large denomination bills are designed to be easily scanned externally since they contain metal strips in the bill design.
Bitcoin main use (regardless of what it was designed for) is for laundering money.
Also it's not a currency, but an asset - as it is designed to appreciate due to how mining works.