Past tenant got email from website stating balance.

A past tenant moved out in July no issue. His unit is rented and he is not  in the current rent roll. He still was able to access that unit through the website and leave a note for me. So... that unit has new tenants, but if you look at the notes it says his unit # and he said the website also says he has a balance. He told me it happened in august also but thought it was a goof because we never called. So he is still linked how? The cloud? Does the new tenant have to register also to update?