Your co-workers must be very insightful in order to "know" that nothing illegal is going on. As for me, I don't believe I fully understand the cycle of funds so I would ask the customer what's happening and go from there.
Your CPA firm is correct; the inability to figure out what the customer is doing is an adequate basis for filing. From the SAR instructions:
the financial institution knows, suspects, or has reason to suspect that the transaction or pattern of transactions of which the transaction is a part:
[abridged]
Has no business or apparent lawful purpose or is not the sort in which the particular customer would normally be expected to engage, and the financial institution knows of no reasonable explanation for the transaction after examining the available facts, including the background and possible purpose of the transaction...
_________________________
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.