First, there are no laws, per se, prohibiting the storage of currency in a safe deposit box. There are, however, the obvious rules against laundering money. If the box is used to facilitate laundering money, and you discern this, file a SAR. A frequent flyer in this area is the customer who visits the box frequently (several times a week) and proceeds directly to the teller line to deposit $5,000 - $9,900 in cash. If this pattern isn't appropriate for your customer (I can't think of a legitimate customer profile it would fit!), you've got suspected laundry and suspected structuring.
Second, many banks employ contract language forbidding the storage of cash in their safe deposit boxes, because of the insurance problems. Customers need to be reminded, of course, that the bank's coverage does not cover the contents of customers' boxes.