Another good story here:
When I worked for Dominos as a delivery driver many moons ago there was another driver there sacked for taking deliveries, taking the money off the customer and then marking the order as void on the system when she got back to store.
She was caught as each manager had their own password, and the password she was using one night belonged to a manager who was on holiday that week. Stood out like a sore thumb on the end of day!
Another area open to fraud is voiding transactions, most systems allow it for the circumstances where a customer starts and order but then changes their minds. Voids should be monitored daily. In this case they were used to negate actual sales, of course at the end of the day, no money would be missing from the till in these circumstances as the EPOS system would not expect to have payments for voided transactions. In addition if you want to restrict voids to manager only, you need to ensure that they don't share passwords.