Mich. Comp. Laws § 338.844

Sec. 24. (1) Each person, partnership, firm, company, limited liability company, or corporation licensed and operating under this act shall make a complete written record of the business transactions and reports made in connection with the operation of the agency. (2) A professional investigator agency that receives or generates a written or electronic report shall keep the report on file in the office of the professional investigator for at least 2 years unless the file is returned to the client or agent.