If the following is notated: "customer wasn't expecting check and paying back won't verify" is this sufficient or what additional facts/details should be documented? This documentation seems weak in my mind... thoughts?

The absence of information doesn't give you any more details about the check than when the customer came to make the deposit. What you have there doesn't constitute reasonable cause. "Other bank won't verify" is never an acceptable reason.

Now if the customer says they won the UK National Lottery and needs to wire money tomorrow to pay the taxes on their winnings, you have reasonable cause. Of course, in that situation as well as yours, refusing to accept the check for deposit is the best option.

I agree with Brian. It doesn't appear that you don't have the "existence of facts that would cause a well-grounded belief in the mind of a reasonable person."

Once the check has already been accepted/processed though, I have been known to on occasion "violate" Reg CC and place the extended hold for a suspected fraud situation using the reason "doubt validity of check". In my role I manage both compliance and security/fraud, so I'd much rather defend a Reg CC violation for just cause to a compliance examiner than explaining why the banks' fraud losses are in the thousands to a S & S examiner. It's a risk I choose to accept.