There have been many cases like this, but I always remember the case of Mt.Gox in 2014, where 850,000 Bitcoins "disappeared" (yes, you read that correctly) and supposedly no one knew where they had gone. The way of acting is always the same, large amounts of money "disappear" without anyone knowing anything... After 7 years, there are still many questions about the Mt.Gox case, unanswered questions.