When I had my store, I had a large amount of merchandise that was being stolen by employees. I know it was employees, as of one point, I was missing 25 easels that were stored in my basement. As brazen as my customers were with stealing, I knew them. I had to face the unfortunate reality, that it was my staff.
There's another form of employee theft that occurs, (which to the best of my knowledge I didn't have) this revolves around "returns".
I asked one of my stores, who was targeted by this, how it works. "We have a customer list in our software system. Typically when a customer makes a return, we look up the customer, see their history, and then credit their credit card. The customers our employee was pulling up, were not really there. She picked random items that were not really being returned. She then did a return by item to her own personal credit card under random names. In our system, you can do a return by ticket, or by item. (She did it just by item)
Previously, our return policy was a staff member could issue a credit to any customer as long as it was within 90 days, and the customer had a receipt. We have now changed out our policy. Two people must sign off on returns."