I would pin a warning note on the bootom of the tree, and would put something in the stand such as a branch or something to make sure there was activity. I have found stands on my own land and have done the same thing. I always give a warning and make sure that the person has read it. If it is still there after that, down it comes with another note letting the person know where they can pick the stand up.
I wouldnt just take the stand for my own for the simple reason that the person whom it belonged to may try to seek revenge or compensation by stealing or damaging something of mine.
Got that right, and very good point. Especially if you are dealing with a secluded area.