The regulatory panel, prompted by the audience, had a protracted discussion on this topic. They did not give a specific time frame or a specific dollar amount for when a bank should consider outstanding cashiers checks to be suspicious. However, they did stress that purchasing then not negotiating an official check was one method of hiding assets from seizure by law enforcement agencies. One of the panelists gave an example of unnegotiated official checks in excess of a million dollars. (Hard to argue with an example like that.)
Trying to fairly paraphrase, the discussion revolved around a bank's responsibility to realize that some large items have been outstanding for an unreasonable period of time, attempt to research the source of funds, and if necessary to unravel the puzzle, contact the purchaser. (Your practice sounds compliant.)
As far a reason for filing a SAR, I'm aware of an FDIC examiner who considers everything inexplicable to be "income tax evasion" (He says that even when the bank is familiar with the source of funds and the underlying transaction was not taxable.)
A more logical rationale is found in one of the filing triggers. The action has no apparent business purpose or the financial institution cannot determine a logical reason for the action. In brief, it just doesn't make any sense. There's no check box for that, but "35z" has a fill in the blank option. Being unable to contact the purchaser would not be a reason for not filing a SAR...
The above is my attempt to accurately relate a conversation by others, not an expression of opinion on my part.
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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.