According to CIP, a safe deposit box is considered an account. I don't see how you could reasonably say that someone who leases a safe deposit box wouldn't be considered a customer.
https://www.ffiec.gov/bsa_aml_infobase/pages_manual/OLM_011.htmPursuant to the CIP rule, an "account" is a formal banking relationship to provide or engage in services, dealings, or other financial transactions, and includes a deposit account, a transaction or asset account, a credit account, or another extension of credit. An account also includes a relationship established to provide a safe deposit box or other safekeeping services or to provide cash management, custodian, or trust services.