The Michigan law that governs wills is called the "Estates and Protected Individuals Code" or "EPIC" for short. The law can be found online at the Michigan legislature's website in Chapter 700 of the Michigan Complied Laws.
Michigan law requires that anyone in possession of the Last Will & Testament of a deceased person must forward the will with "reasonable promptness" to the probate court having jurisdiction by delivering it personally or sending it by registered (certified) mail. Failure to do so may cause the person in possession of the will to be liable for damages, or held in contempt.
But, you may ask, my loved one had a properly funded trust agreement, and there appears no reason to open a probate estate! Not to worry - the act of forwarding the Will to the Court will not, in itself, subject any part of your loved one's estate or assets to probate.
For a directory of local probate courts, visit my website.