Suspect deposit into account

On 4 Jan a fairly large sum was deposited into our cheque account but we don't know who deposited it. Nedbank's contact centre was offline twice when we called, so we tried our local branch. No feedback to date - and that after 2 phone calls (left messages) and 2 emails to the Branch Manager. Please assist if you can - someone could be waiting for this money. All we want to know is who deposited this money?