In my state it is ILLEGAL to use an unmarked stand regardless of who owns it, and I think it may even be illegal to use another persons stand. I have a hard time thinking you'd get dinged for it, but technically it may be illegal and you never know--best to check regs in your state. Also, hunters here are required to have to have name and address clearly readable on the stand, and on most public land have to take their stands down a couple weeks after the season is over, cant have them up until a couple weeks before, to avoid having a stand left in the woods be a "territory" that one hunter claims, or for the stands to become unsafe litter left in the woods that prevents others from accessing the area as they might have otherwise. If your state has similar regs it may very well be that you are seeing stands illegally left in the woods, which really galls me since they are essentially claiming an area outside the rules that are set specifically to prevent any one person from claiming an area on public land. I think leaving stands up like this outside the hunting season is really lame and truly inconsiderate.
Personally, If its a spot I like I usually call the hunter if the stand is marked and just touch base--never had anything but a good response to this--although I've never asked to use their stand, I have often been told that they only hunt the rifle season and that outside of that time they definitely wont be there, and so I've hunted nearby a couple times if the location is good. once I talked to a guy who was clearly going to be using it a lot, in which case I probably didnt want to be there anyway. However, I would say 95% of the stands I find are completely unmarked and most appear like they've been there for a couple years. I mark them on my gps and more or less ignore them unless in-season I see a car or some reason to think it is in use--9 times out of 10 those stands when I find them dont get used much (or at all), and I have yet to see a person using one of them. I saw a great idea (maybe here?) that I've been using for a couple seasons. I now will hang a fallen branch on the ladder or steps, so if someone uses it they have to remove the branch. That way if I go by again and the branch has been removed its likely someone is actually using the stand at least occasionally. If I'm hunting an area and know there is a stand there I'll be real careful not to screw up someone else's hunt, but since so many of these are just abandoned around me that most times they just get a careful look and then ignored once I can determine no-one is in it.