&gt; as long as he didn&#x27;t keep logs.<p>It doesn&#x27;t matter who keeps the logs, the blockchain is the log.  I really think you are the one who is missing something.  Because the blockchain has the entire history, everyone knows every transaction wallets X, Y, Z and any other wallet has ever done.<p>&gt; A block chain is a transaction database shared by all nodes participating in a system based on the Bitcoin protocol. A full copy of a currency&#x27;s block chain contains every transaction ever executed in the currency. With this information, one can find out how much value belonged to each address <i>at any point in history</i>.<p><a href="https:&#x2F;&#x2F;en.bitcoin.it&#x2F;wiki&#x2F;Block_chain" rel="nofollow">https:&#x2F;&#x2F;en.bitcoin.it&#x2F;wiki&#x2F;Block_chain</a>