Many people don't care about opsec. As for what the cops can do... international arrest warrants on both him and the money are one thing. I can easily think that the German government has notified the exchanges about the public keys and that any attempt to trade coins originating from them would constitute some anto money laundering guideline.

Besides, there is no statute of limitations on seized assets. If anything, he'd commit a new crime (theft of government assets) by accessing them.