From the exam Manual:
Efforts to Avoid Reporting or Recordkeeping Requirement
•A customer accesses a safe deposit box after completing a transaction involving a large withdrawal of currency, or accesses a safe deposit box before making currency deposits structured at or just under $10,000, to evade CTR filing requirements.
As suggested, if using the safe deposit box was an element of attempting to structure, you have the basis for a considering a SAR. That's not what you have here. All you are left to consider is whether it is suspicious that someone keeps cash in a safe deposit box. Hint, it's no more suspicious that they keep it in a safe deposit box than if they keep it under their bed.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.