From the Interagency Flood Q&A #40:
If a loan is secured by Building A,
which is located in an SFHA, and
contents, which are located in Building
B, is flood insurance required on the
contents securing a loan?
Answer: No. If collateral securing the
loan is stored in Building B, which does
not secure the loan, then flood
insurance is not required on those
contents whether or not Building B is
located in an SFHA.
You may also want to look at Q&A #39: https://www.fema.gov/media-library-data/20130726-1742-25045-4927/interagency_q_a.pdf.
As a lender, you may want to consider requiring insurance, if available, on your collateral even if it isn’t required by the flood rules.