Here's what I came up with from the Michigan Compiled Laws. There's a lot of legal mumbo jumbo to wade through, but essentially it says you can't be there if it IS posted or fenced, and you can't be there if it's NOT posted or fenced.
324.81133 Operation of ORV; prohibited acts.
"Failure to post private property or fence or otherwise enclose in a manner to exclude intruders or of the private property owner or other authorized person to personally communicate against trespass does not imply consent to ORV use."
324.73102 Entering or remaining on property of another; consent; exceptions.
(1) Except as provided in subsection (4), a person shall not enter or remain upon the property of another person, other than farm property or a wooded area connected to farm property, to engage in any recreational activity or trapping on that property without the consent of the owner or his or her lessee or agent, if either of the following circumstances exists:
(a) The property is fenced or enclosed and is maintained in such a manner as to exclude intruders.
(b) The property is posted in a conspicuous manner against entry. The minimum letter height on the posting signs shall be 1 inch. Each posting sign shall be not less than 50 square inches, and the signs shall be spaced to enable a person to observe not less than 1 sign at any point of entry upon the property.
(2) Except as provided in subsection (4), a person shall not enter or remain upon farm property or a wooded area connected to farm property for any recreational activity or trapping without the consent of the owner or his or her lessee or agent, whether or not the farm property or wooded area connected to farm property is fenced, enclosed, or posted.
(3) On fenced or posted property or farm property, a fisherman wading or floating a navigable public stream may, without written or oral consent, enter upon property within the clearly defined banks of the stream or, without damaging farm products, walk a route as closely proximate to the clearly defined bank as possible when necessary to avoid a natural or artificial hazard or obstruction, including, but not limited to, a dam, deep hole, or a fence or other exercise of ownership by the riparian owner.
(4) A person other than a person possessing a firearm may, unless previously prohibited in writing or orally by the property owner or his or her lessee or agent, enter on foot upon the property of another person for the sole purpose of retrieving a hunting dog. The person shall not remain on the property beyond the reasonable time necessary to retrieve the dog. In an action under section 73109 or 73110, the burden of showing that the property owner or his or her lessee or agent previously prohibited entry under this subsection is on the plaintiff or prosecuting attorney, respectively.
(5) Consent to enter or remain upon the property of another person pursuant to this section may be given orally or in writing. The consent may establish conditions for entering or remaining upon that property. Unless prohibited in the written consent, a written consent may be amended or revoked orally. If the owner or his or her lessee or agent requires all persons entering or remaining upon the property to have written consent, the presence of the person on the property without written consent is prima facie evidence of unlawful entry.