First of all a bank should not know what is in a safe deposit box.
That would be my feeling as well. There was another thread about keeping a firearm in a box. Just as in that case, I don't think that the bank should be inquiring as to the contents of someone's box. Also, customers should be provided with a private area to access the contents of their box.
The only situation where I would file a SAR about cash in a customer's box would be if we had reason to suspect that they were keeping it there for an illegal purpose (tax evasion, etc.). I know a few people who keep a decent amount of cash in their boxes as a form of diversification & liquidity. Unless there is a
specific reason to suspect that the cash was being stored there for illicit reasons, I wouldn't file a SAR.