Each ATM communicates on line with the bank that services the unit. When the "reserve" cash reaches a certain level, the bank schedules a refill. If nobody ever uses the ATM then it never gets refilled. The bank will not give out the information about how much money is kept in the ATM or how much is in there when it triggers a request for money.
I imagine there are sensors, monitors, or counters that keep the main bank notified, and they refill when it gets down to a certain level, and that depends on average use, as tracked over time.
awww come on now we all know your trying to rob the next refill.