I think the card was used in Grafton? So probably a new patient. I guess it depends what she was going there for, as to how deep into her identity they would go. If it was an ongoing medical issue, she may well have had records from somewhere else?
If it was the bank in Byron, I would agree completely, but it was the colonial in Ashmore, where she lived- where she held an account no one knew about. There is a reasonable chance she knew the person who identified her. She was withdrawing the balance of the account- which could have been any amount, we just don't know. For me the fact that they were able to contact her later by phone suggests that at the very least they had updated her contact details- wouldn't it be good to know what that number was! There wasn't a huge amount of time between when Sally reported her mother missing and the bank having to remember. The bank sighting seems to be the basis for the initial identification as located.
I wonder why she had that separate account? She wasn't married, so its not like she had a secret stash hidden from her husband. Could she have been holding money for someone else?