As this story shows, people are an important fraud protection tool for retailers.
You should not under-estimate the power of motivated staff to stop or report fraudulent actvity.
In order for this to work you should consider these:
1. Do your staff have a confidential way to report potentially fraudulent activity such as an outside whistleblowing hotline like Expolink? You may fall back on the old line that they should report it to their line-manager, however you should consider that collusion is often found in retail fraud and that the line manager may be in collusion with the activity and therefore take action to protect their position. Which may give your business the double-whammy of suffering fraud and then firing honest staff who tried to prevent it! This is one area where lines outside of the business makes sense.
2. It is probably not appropriate to offer rewards for this activity, afterall you don't want to incentivize mis-reporting against unlike bosses, however it is still important to acknowledge how difficult it is for someone to report a colleague for potentially fraudlent activity. If they get it wrong they are possibly putting their own job at risk, so low-impact recognitation is appropriate. For some wider discussions on rewarding whistleblowing see here.
What do you think, how would you reward a whistleblower? Afterall its probably easy to define the loss and therefore the predicted loss if you were not made aware of it. If for example someone generated an extra £2000 income, you would probably reward them, so why not if its preventing a loss?