I'm assuming your initial hold period was based on a "new account" exception hold. Any hold you impose right now would be based on the facts and circumstances surrounding the transaction, not a reference to specific law or regulation.
In your shoes, I would call the customer and ask "What's the deal?" Depending on what explanation is offered, I might then call the issuer of the check.
Please don't misunderstand, I am not saying I would not put on a meaningful hold (perhaps based on reasonable cause to doubt collectibility) I would just gather all the information I could first.
"Oh God, thy sea is so great and my boat is so small." Breton fisherman's prayer in JFK's office.