Our party had similiar occurances. Actually, this has happened to a few of my permanant stands. We used to hunt on public land between Mille Lacs and Hinkley. We were tucked way back into the woods, much deeper than others were willing to go, so we thought. My first year deer hunting, i was walking out before sunrise to my perm, got there and it was gone!!
Looked at it after sunrise and found the nails on the ground and wood scattered around. Two years later, i had a great perm, on a hill looking down at a creek. I get two real nice deer on opening weekend.
The next week, head out again, NO STAND!
Was torn down again. We have always had success up there, but the parties near us do not. So i’m guessing out of jealousy, they tore my stands down. We have also brought up portables just for easy mobility, because the state has been logging out the woods, and deer have been moving around a lot. First year, we have a stand stolen during the week after opener. So we learned our lesson, and i welded chains to the stand and padlocked them to the trees the next year. This worked better, but unfortuantely after the second weekend, the chain was cut and the stand was gone. We had an idea who took em, but there was no way to prove it. 
Since then, we gave up on these woods, and are now hunting in Park Rapids on my uncle’s property. Prime land and is surrounded by state land, so don’t have to worry about any thieves!(besides his driveway, it is atleast 10 mile hike through swampy, hilly, state woods from any road) 