Coinbase executive: "14-year dormant whale 8 billion USD BTC transfer" may involve private key leakage

Coinbase director Conor Grogan said in a social media post that there was a possibility that the private key could be stolen or leaked yesterday "after 14 years of dormancy.
Conor found that one of the wallets made a test transaction of Bitcoin Cash (BCH) an hour before the massive transfer of BTC assets by the Whale Wallet and then transferred the entire amount of BCH. Conor believes that this pattern of behavior is unusual. Other BCH wallets are not used passively, and BCH is usually less noticed by the Whale monitoring service. It may be a low-key way to test private keys. He speculates that these wallets are unlikely to be exchange wallets. Conor said that while this is just speculation, if true, this could be the largest case of digital asset theft in human history.