I’ve recently been involved in some conversations about riding bikes on trails which connect to designated park-owned mountain bike trails, but lead off of park property on to other lands. As a result I decided to research trespassing laws in Michigan, specifically the Natural Resources and Environmental Protection Act (Act 451 of 1994), Part 731: Recreational Trespass. In particular, 324.73102 states:
(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.
Since we all live in a land where everything which is not prohibited is permitted, I read this as an assurance that entering property of unknown ownership, for any recreational purpose is permitted so long as one does not cross a fence designed to keep people out or pass a sign prohibiting entry. While on this property one is not permitted to modify the property, as per 324.73109 there are penalties for property damage.
Therefore, I take this all to mean that riding from trails on known park property to trails on unknown, unposted property is wholly permitted, so long as one does not modify the land (read: build new trails or other stuff) or cause damage.
UPDATE: As Cefai noted in the comments below, this is consistent with section 750.552 of the Michigan Penal Code which states:
(1) A person shall not do any of the following:
(a) Enter the lands or premises of another without lawful authority after having been forbidden so to do by the owner or occupant or the agent of the owner or occupant.
(b) Remain without lawful authority on the land or premises of another after being notified to depart by the owner or occupant or the agent of the owner or occupant.
(c) Enter or remain without lawful authority on fenced or posted farm property of another person without the consent of the owner or his or her lessee or agent. A request to leave the premises is not a necessary element for a violation of this subdivision. This subdivision does not apply to a person who is in the process of attempting, by the most direct route, to contact the owner or his or her lessee or agent to request consent.
Thus, unless you’ve been told not to enter the property or to leave, you may do so, unless it’s a posted farm. Then you may only enter to seek permission to remain.