Your situation is probably different then what Im about to explain, but if not, take note.
A friend and I got permission to hunt 20 acres this year. We hunted most of the season then found a note on a stand that has been there since we showed up(home made wooden). Next week that stand is gone. Then my friend has his bow rope stolen, and a set of rattling horns. As luck would have it, there were 4 wheeler tracks in the snow. We followed them to the house. Turns out the guy was real nice about taking the stuff, and said he had just bought the property, and he thought we were on his property. We told him we will be back later to sit down and discuss the property lines, as we have done extensive research with plot maps and areial photos.
If you have owned the property for some time, and they are well within your boundry, then I would take them and leave it at that. They wont know if it was you, or another tresspasser. If they ask you, there is your chance to make it clear its your property, and you can determine if you want to admit you have their stands or not.
Kenn