I don't know where you're detecting but south of the Mason Dixon line when we find a heavy skeleton keys on a colonial farm it's usually the key from the slave lockup. There again, I don't approve or glorify in any way that chapter in American history but the artifacts speak for themselves. I detected the yard of a colonial home in Clarke County, Va about five years ago. It belonged to my wife's cousin and her husband. I found some Mercs and Rosies and a silver nickel, a pocket watch (FUBAR) and a huge skeleton key nearly longer than my massive paw. It was surrounded by other misc iron artifacts that didn't lend a clue. Every plantation had a lockup! Those folks were so afraid, after Nat Turner, that they couldn't lock up enough to make themselves feel secure. Wife's cousins husband, from a successful Texas plantation couldn't rationalize what I told him, but he hung the key up in his kitchen. He told me he would get another opinion on the artifact with a nervous smile. He never told me the result of the second opinion but the key no longer hangs in his kitchen! His home still had the hand forged colonial lock on the original front door. The key I found was way too big for that.
We lived it, we own it, lets find out as much as we can about it!