Depends on the credit union, some are listed on the ICB.
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Some are not. If listed on the ICB then the "banks will know about it".
Your deposit should be from savings or a gift. If there is a large cash deposit into your current account, the bank will question where it came from. Adding to what allthedoyles has said, if you repay by direct debit then these repayments will also show up on your current account and the bank will also question this.