Unauthorised debit orders - Mum's the word

I have noticed a "custody fee" debit order being deducted from my current account. I went in to my branch to clarify, the response I got was that they could not tell me who authorised it(i always thought the account holder would authorise any debit...) or what it was for. They were able to reverse the deduction, but unfortunately though now I need to "Sleep with one eye open" just in case they dream up any more unauthorised debit orders.