I have wondered about customers getting cash back on automated checkouts in supermarkets forgetting to pick it up, going back a few minutes later and it's gone.
I'm sure the staff don't just pick it up. 
If someone like my self, or workmate see cash back left by the customer, we take it to the customer service desk, and log it. So if the customer comes back, there a good chance they will get there cash back
Now a warning for thieves, who think there £50 cash back, left by the customer, I will just take it for my own needs. Worst thing you can do, especially over the last 12 months cctv in these area has been increased.
Customer returns and asks us did anyone hand in £50 cashback, no, we now take this to security. Security check cctv, and see the person who took the cashback, and now this is sent to the police. The police with in a few days have identified the person, and knocking at there door, and cautioned and arrested. 15 persons locally to me have been arrested for theft of cashback, and some of them never even been in trouble with the police, and now have a criminal conviction