Have you ever had a problem with a machine and asked for a slot tech? They pull out your card and insert theirs before they open the machine and the box probably signals who is working on the machine. I would think this is a security feature so that if a machine is opened without an authorized employee card inserted, all kinds of red lights would go off in the security office. Every so often I have seen a employee card forgotten on a machine, but it happens a lot less often than with standard cards.