It is not the bank who will be asked. But could be asked by some other government agencies.
But do not worry, then you have in the trading program the history of your account for all transactions from the deposit , withdrawal , profit and loss can be printed notation of history by the printer and submitted to the concerned people so as not to charge money laundering or receive money from any illegal way.
Do not worry Forex Trading is a hundred percent legitimate
Even countries and governments sometimes resort to it to provide foreign currency.