Cash in a safe deposit box certainly should trigger some enhanced due diligence, but it does not justify an automatic SAR filing. The problem is that, since you have no other relationship with this person, you are not in a position to do much investigating. One of the "red flags" mentioned in the BSA/AML Exam Handbook is:
Safe deposit boxes or safe custody accounts opened by individuals who do not reside or work in the institution’s service area, despite the availability of such services at an institution closer to them.
Just like it's a poor practice to sell official checks for cash or send or receive wire transfers for non customers, it's an equally poor practice to rent them a safe deposit box. Many banks simply will not do it.
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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.