what a bummer, i am so sorry to hear your story. we had a similar issue where i work several years ago. We had an employee actually ring up the items, tell the customer what the price would be (the $ was on a digital screen for the customer to see- and we have a touch screen POS system), but after telling the customer the price, the employee would then delete the items, line by line, cash out the ticket, open the drawer- take the customer's money and put it in the register.
apparently this employee would 'save' up the receipts and add up the booty later when counting down the register at the end of shift, then pocket the cash- or family would come through the line during the day and would receive an extra portion of change.
it was a sad, sad deal.
the employee was discovered basically through a hunch. accounting personel began looking through daily transactions and noticed all the voids- the sting operation took place soon after.
now, voids are regulated through reason and signature. every employee *has* to login with their own user name and pin, receipts must be given out to every customer and we have cameras- fortunately- no one is watching every move that is made, but requests can be made to upload film from two weeks ago at approximately 3:10 p.m.
I hope you understand, I was not saying your were sloppy. You really have to stay on top of things like this to eliminate loopholes for this kind of thing to happen. It's not easy.