Alex of Inside NEM brings clarity, explaing what is really going on.
She said the stolen NEM will be Marked!
NEM/XEM is a public blockchain so anyone can look up the NEM block explore to see the transactions provided they can get all the exchanges on board to not accepting those stolen XEM.
Those stolen coins are tainted.
Ep 32: Coincheck Exchange Hack Recap + Update
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https://blog.nem.io/nem-protocol-tracks-funds-stolen-from-coincheck-exchange/