Safe deposit contracts oftentimes prohibit storing currency in the box. Obviously, the safe deposit custodian would have absolutely no idea if the renter violated the restriction unless there was a "mysterious disappearance" and the renter alleged that cash stored in the box was now missing. (Easy to allege and hard to disprove.) The contractual provision gave the custodian a "pat" reason to turn down the claim: i.e. Maybe there was cash your box, maybe there wasn't. Either way, we told you not to store cash there so we have liability.
The language and the rationale predates the Bank Secrecy Act by a hundred years or more.
Obviously, some customers keep cash in their safe deposit box and they are goofy enough to make bank personnel aware of it. The FFIEC BSA/AML Examination manual contains 1 or 2 examples where the activity might be suspicious.
There are more important things to drill into your personnel.
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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.