On state land in Wisconsin, stands must be removed at the end of the hunting day. They may be left up on private land. On county owned land where I hunt, the county premits two stands to be left up during season, but does require that your name and address is on them.
If you don't have an identifiable marker on your stand, an engraved name or something like that, it's difficult to prove the stand is yours. I've never found another hunter in one of my stands, probably because I hunt pretty far in. However, if I did find another hunter in my stand, I would not hesitate to ask him to vacate it right now. I wouldn't try to stop him from hunting in the area. He has as much right to hunt there as I, but not out of my stand. I usually chain my stands so they can't be opened and sat in. However, after an episode in which I found another hunter in my stand, I would pull it and go elsewhere.
One might get the idea that the same stand hanging in the same place for two years was just left (Believe it or not, people do that.), but signs of use should have been sufficient to indicate that yours wasn't abandoned. Some slob stole it, plain and simple.
I chain and lock my stands to the tree, but I've pretty much just been lucky to this point. If someone finds one and wants it, chains can be cut. The only way to guarantee someone won't steal it is to take it with you.
Unfortunate, but that's the way the world is nowadays.