Could readiness hints warn Russia without disclosing the operation?

Could careful wording and timing communicate preparations without explicitly revealing the operation?

Possibly, after enough correspondence established a relationship, but enemy analysts would also be watching for those hints.

A regular exchange of letters could create enough familiarity for the admirals to convey an indirect warning. A discussion of increased readiness and keeping boilers going might suggest that ships should be ready to sail at short notice. Such wording would still have to avoid revealing the operation itself. Enemy intelligence officers dissecting the correspondence could recognize the same implications, so the method would offer no guarantee of secrecy.