I think the point is you know that beaches don't have diamonds in them in reachable range because people frequent beaches. But it's plausible to me that the CIA

I think the point is you know that beaches don't have diamonds in them in reachable range because people frequent beaches. But it's plausible to me that the CIA is sitting in unique operational territory, and their informational "obviously worthless" periphery is not well frequented. It's not about finding diamonds on the beach, it's about finding beaches with diamonds.