Does my ex-spouse have to know I hired a lawyer on a limited scope retainer?

A lawyer is required to avoid acting such that it appears the lawyer is providing services to the client under a full retainer. If the lawyer deems that disclosure of the nature of your retainer is necessary to meet this requirement, then the lawyer will seek your instructions to disclose this information to an opposing party and/or their counsel. Such discretionary disclosure may also be necessary where the lawyer represents you in court.