What is happening under the table in your shop. Well, one point of a functional point of sale system is that the numbers never lie. That is why you must regularly check your information and scrutinise your figure to highlight any inconsistencies.
So I suggest that you regularly check your Retail Protective Key Performance Indicators, to see what people are entering into the system.
These indicators give you a detailed breakdown for each operator on no sale, voids, count, totals, refunds, negative items and discounts. One figure I suggest you look at in particular is the voids.
To see them, go to cash register reports. (see orange arrow)
Now select Staff and click on "Sales security indicators" (see blue arrow)
Select the required period, I recommend doing it monthly, quarterly and yearly.
The most successful internal thieves in my experience have these characteristics
1) They have patience
2) They take only a little at a time.
3) Keep a low profile
4) They do not tell or share with others when they do it can be an absolute disaster for the business.
Generally what happens is that develop a method which works for them, and they tend to use it methodically. Approximately $80 a day assuming they work 250 days a year gives then $20,000 tax-free.
This creates an item that is way above average on some item. This is what you with this KPI check is looking for is some item way off the scale eg $20,000 of voids a year.