Mt. Gox, once the world's biggest exchange of bitcoins, says it has found 200,000 of the virtual currency in a digital "wallet" that was no longer used by the company. That represents about a quarter of the bitcoins that the company had announced were lost when it filed for bankruptcy-court protection in February. "We believed there were no bitcoins left in old wallets, but found 199,999.99 bitcoins on March 7," CEO Mark Karpeles said Thursday. The company still has not said how exactly the bitcoins, valued at roughly $470 million, went missing, but did say it was possibly a technical issue allowing users to make fraudulent withdrawals.