Cone of silence

From the old TV series, “Get Smart.” You remember, right? Secret agent Maxwell Smart would meet with his colleagues under a large glass dome so that only the party he was talking to could hear what he was saying.

Use: “HR Director Donna called her friend Lori, an HR director at another company, to verify a job reference. Lori gave her the straight story, but told her to keep it under the cone of silence, as the company wasn’t supposed to release that information.”