We don't know where on the trunk these boards came from ( hi up or lo down ) but the most suspecting area would be 2-6 ft. up from the ground . That's most likely where something was nailed like no -hunting signs , barbed wire or if in someone's yard some sawyers may not want them at all .
As mentioned use the magnet or a metal detector . The sawyer i'm acquainted with , on a log , he looks at the end for black stains when it's still in log form . If so it's chain saw time making fire wood .