A customer sells a car to someone who pays for it with a fraudulent check and the teller is going to refuse to give the check back, leaving the customer with no proof of the fraud? If the customer is rlcarey or me you just threw a 19 year old teller to the wolves.
Having a policy where the bank retains the check "if appropriate" might work if it's administered by people who know enough to ask the right questions. The bank does not stand in the place of a parent who can confiscate dangerous toys.
In your example, I would likely change my stance on it on a per-case basis. But I've never seen anyone actually lose property like that. Most of the scams have to deal with the fake lottery winnings, etc. Customer brings in a check, deposits it (if it gets by a teller), deposits it in an ATM, etc.
So yes, in those cases we keep the original items once we find out they are fraud. The customer is more than welcome to a photocopy. We especially do not hand the check back if the customer brings it in and we refuse to deposit it because we know it's fraud. I am not giving back a fraudulent check to a customer who can then take it to another branch or another financial institution and potentally be allowed to negotiate it.
If the customer would need an original because it was required by law enforcement, that's a different story. But we've never had anyone need anything more than a photocopy in order to file a police report. And most people don't even file police reports because a) they're embarrassed that they fell for or almost fell for a scam; or b) they didn't actually suffer any losses because the funds deposited never left the account prior to the fraud discovery.
Our fraud prevention policy does state that bank employees are to retain the fraudulent item and any exceptions have to be approved by our VP of Fraud Prevention or myself.