The customer deserves your attention for a while. Review past activity. Review future activity.
The teller deserves an "attagirl" or "attaboy" as appropriate from your CEO or at least from you...
There is a checkbox on the SAR, Item 29(f) Suspicious inquiry by customer regarding BSA Reporting or Recordkeeping Requirements. If in looking at past or future activity you found evidence of structuring, then you would check that box as well.
However, asking a question is not "structuring" and, by itself, asking a question is not the basis for a SAR filing.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.