To recreate: Have StoreLastBorrower enabled to store Last Returned By. Create account and leave cardnumber empty. Check out and return item with that account. Go to items detail page. See that Last Returned By and, if return is not anonymized, Last Borrower appears blank (you can inspect the fields and see that borrower information is there but not visible). Previous borrower will similarly be blank if it is from an account with no cardnumber. Most libraries will usually have a value in cardnumber, but if they don't for any reason, perhaps for in-house cards or by user error, it hides important circ information.