Some states are different...but in PA, you DO NOT have to have your land posted. If its not your land, and you don't have permission, then your tresspassing!
I would contact police (its a civil case) unless the person hunts the stand and kills something...then contact the game comm. (its trespassing and poaching then).
However, I would personally take the stand down...no note....nothing. I'd be sure to be in the area well before first light in my stand and wait for the person to show.
What? You had a stand here...haven't seen it! BTW, do you have permission to be here? LOL