If a user doesn't have an outstanding balance, the UI doesn't show this (to avoid unnecessary confusion) -- which is probably why the user doesn't see it now (because the unpaid auction was deleted and the user's account credit was charged $100 for non-payment).
Aside from the UI displaying this information, when an auction closes, the winner receives an email that notifies him to add credit to his account.
Additionally, the system emails every user with an outstanding balance multiple times before the 2 week payment period expires (after this payment period expires, the auctioned domains are released to be reauctioned, and the user is charged a $100/domain fee for non-payment).