Just a thought...Do we know if the patio in the back of the "current" house has always been there. If I had the addy, I could check the prior real estate listing. Just looking at areal pictures of the house, it caught my eye as a possibility...
If they probed and not dug up the back yard, it would explain why they didn't find anything as you can't probe concrete...Just throwing it all out there and maybe we can all help figure this thing out!
If you do a word search for "deck" in this thread you should find a couple messages about it. As I remember it, someone thought it was new because it looked their own which they had recently been working on.
The well publicized address was 218 Miller Chapel Road in Salisbury, Rowan Co, NC
which sold on 18 Apr 2011 to grandfather PARSONS.