From the story it is possible that he had withdrawn the money from the Banks or ATM's previously, they were laying in wait near where he was staying and caught him as he was using an ATM - this might have been just a few baht before going to 7/11, not the million mentioned in the story.
I guess it didn't occur to the neighbors to go knock on the door and see if she was okay, the 500 baht bounty for snitching must have been too tempting!