Although Digicash's ecash offers anonymity to the payor it does not to the payee. The reasons have to do with the way coins are blinded. So LE could, with the bank's cooperation, easily associate the two sides of a transaction. This was intentional on Chaum's part, either for moral or practical political considerations. Its probably only a relatively minor patch to allow one ecash purse (the kidnapper's) to generate the blind token values so that another (similarly patched) purse (the vicitim's) can submit them to the mint and return the minted coins to the kidnapper (e.g., by posting on a popular Usenet group).
Probably more than a minor patch, but doable nonetheless.
In this scenario the only reasonable way left to track the money is via linkage (the size and timing of deposits and withdrawls in the kidnapper's account).
I can't see any reasonable way to track money obtained using the double blind protocol - after all, the kidnapper does not even need to have an account. Best regards, Gary -- "Of course the US Constitution isn't perfect; but it's a lot better than what we have now." -- Unknown. pub 1024/C001D00D 1996/01/22 Gary Howland <gary@systemics.com> Key fingerprint = 0C FB 60 61 4D 3B 24 7D 1C 89 1D BE 1F EE 09 06