We monitored a customer's activity after a suspicious question/comment about the reporting threshold that he made to one of our officers. The officer was certain the customer would be structuring a transaction in the future based on his comments, but there has been no structuring or anything out of the ordinary in his account before or since that conversation. No doubt, we will be monitoring his account from now on. My question is, should a SAR be filed based solely on an inquiry without subsequent suspicious activity?

Although 29(f) on the SAR is labeled: Suspicious inquiry by customer regarding BSA reporting or recordkeeping requirements, we have not gone so far as to criminalize speech (at least not this kind of speech) in the U.S.

A question or a comment such as this is the prompt for some enhanced due diligence (just as you are doing). Without more facts, it is not the basis for filing a SAR.

No SAR would be required because the dollar threshold has not been met. Voluntary reporting would not be prudent, in my opinion.