Let's say you were suspicious of a business customer in the past and at the time of the suspicion the Acct Officer was very knowledgeable of the the customer and their business activities and therefore no SAR was filed. Well, a little over a year later you receive an IRS summons, however the account is now closed and the activity you were suspicious of occurred more than 1 year ago. Would you file a SAR now just based on the receipt of that summons?

You would not file a SAR based on the fact that you receive a summons/subpoena. That should trigger a review, but not automatically trigger a SAR.

Those were my thoughts as well, but wasn't sure if I should go back now and file on the activity I found suspicious a year ago or just let it go because it's been so long? My suspicion a year ago was possible tax evasion and when we received the IRS summons, I thought it was interesting.

If your review uncovers activity that you believe is suspicious irrespective of the subpoena or summons, you should file. And when you do, make sure you note in your file that you had a reason for not filing last year (an explanation that turned out not to be accurate).

Agreed. You would not be filing because of the summons. You would be filing because a review prompted by the summons caused you to reconsider the situation in a different light. You are not prohibited from mentioning the summons in the SAR if you decide to file it.

FYI, the IRS does not use its summons authority for incidental income tax disputes. They are thinking about going after your former customer with a meat axe.