Ethics dilemma, or conscience dilemma? My conscience would never allow me to keep those stands. Even if the stands were legally now mine. If they called for them back, I would tell them no problem come get them, and be done with it.
Here is what I would have done in the first place. Not going to lie, I would have taken the stands down but not call them if they had my number. I would leave it up to them to call back for them. If I did not live in the area, I would leave them with my father-in-law. If they did not call for them for awhile I would probably use them myself, but as soon as they asked for them I would feel obligated to give them back. Even if I took them with me and I did not live nearby. I would bring them back to the father-in-laws and let them pick them up there. I would also apologize and tell them I was not sure if they were ever going to come back for them.