They cover this a bit in the NYT article. The system knows if you put something back—or rather it seems that it makes sure it always knows what’s in your posses

They cover this a bit in the NYT article. The system knows if you put something back—or rather it seems that it makes sure it always knows what’s in your possession. I don’t think it’s necessarily looking at the shelves as much as it’s constanly trying to determine what objects are associated with each person.