You do not say how old your sister was, if over 55 she may have been considering a Lifetime Equity Release which would explain the fact that there were no repayments leaving her Bank Account. However I would have expected you to have found annual statements showing an account number if she had gone ahead. Follow HC's advice re Land Registry. If nothing there it would be safe to assume that she did not go ahead, a reference number is just a way of making sure no correspondence etc is ever confused with another party. At the end of the day she may have decided she did not like the terms of the product.