It's the crypto that moves into and out of a wallet that is traceable on the ledger (aka the blockchain) so it's the "spending" of crypto in a wallet that makes it traceable, not the existence of a wallet itself. That's true even if one uses a mixer/tumbler.
It sounds like the kidnapper wants millions of dollars worth of crypto, but it might be interesting to see how kidnapper behaves if their wallet was sent for example $20,000 - would the kidnapper leave $20,000 there forever or start moving it which provides clues for investigators. Even if they left the bitcoin untouched in the wallet for years, it's easy to set up an alert to monitor a wallet and if it ever started moving out of the wallet that would provide clues.