What sort of disclosure should we provide when an investigation begins?

Q: “Often an employer needs to conduct investigations not strictly related to harassment and discrimination. My understanding is that those investigations require a disclosure, i.e. the company states there will be no retaliation, and confidentiality will be assured except in need-to-know situations. But if we are investigating a policy violation for example, or theft, are these disclosures required, and if not, is it a best practice regardless?” – Anonymous, New York