While there may not be a safe storage law, per se, the Michigan State Police believe that someone can be held criminally and civilly liable if they allow a minor to gain access to a firearm.
https://www.michigan.gov/documents/m...F_286476_7.pdf
You may be criminally and civilly liable for any harm caused by a person less than 18 years of age who lawfully gains unsupervised access to your firearm if unlawfully stored. As such, a trigger lock, gun case or other device designed to prevent unauthorized access to a firearm is strongly recommended.
Or maybe that's just some kind of generic warning?