When I was hired as a cashier before, our supervisor told us that we should keep in mind that after every transaction, we must close the cash drawer. This is so to prevent theft and because we were still using the old cash registers. That was 11 years ago.
Today, if you forget to close your cash drawer, are red sign flashes on your screen, so the cashier is immediately reminded. Aside from that, they are automatically recorded on your supervisor’s computer. So even if she/he is not looking, she/he can reprimand you for that. Think about how little things go high-tech nowadays!