If the machine is connected to "home" (or plans to be) then it can phone home whenever the door has been opened. The audit computer can then compare these times with the filler's assigment/schedule.

If the machine isn't in constant communication, but is polled, it can still tell "home" when the door was opened, during the next call.

It's more complicated, but there's no hardware, and it's independent of the filler's attentiveness.