Well no. If a customer pays with cash, a cashier can take payment for something without actually ringing it up. The sale never enters the computer system. Even if you audited the shops inventory, you'll nearly always discover some products are missing, but that doesn't prove employees have been stealing; shrinkage has a variety of causes (shoplifting, damage, was never actually delivered, etc.)

Regardless of theory, pay attention to where cameras in corner shops are pointed and I think you'll see what I've often seen: the cameras are primarily positioned to watch employees.

Or not take payment for a few items that a friend ‘buys’, take payment for 5 items while a friend ‘buys’ 6, etc.

To be fair, registers aren’t used exclusively by one employee. At grocery stores, when someone goes on break, someone coming back might take their spot. That happens multiple times a day. And for restaurants, almost every employee uses the same few registers.

The only way to know who is stealing is to either watch the drawer or to count the cash every time the employee walks away. Of course, most places go with the former as the latter is too time intensive (especially during a rush). The better solution (IMO) is to just count at the end of the day, and if there’s a discrepancy, mark it down. If it happens multiple times, you can compare who was at that register for each of those days and narrow it down.

> The only way to know who is stealing is to either watch the drawer or to count the cash every time the employee walks away.

At least in the grocery stores I frequent (Bay Area), the employee takes the drawer with them when they walk away (usually shift change) - clearly another option. I think they sign in/out too, so the drawer and the transactions are connected.