I think that here in WI, trespassing is trespassing, posted or not. Putting up signs is just a "convienience" to let others know where they are.
Under Wisconsin law, you are not required to post “No
Trespassing” signs or identify your property boundaries
unless your land is surrounded by or borders public lands
like a national forest, state wildlife management area, or
county park. Any person who enters your land or stays
without your specific permission is considered to be
trespassing (Wisconsin State Statute 943.13 1997).