We probably need to get DSpear456 to be a little more clear about what he meant by behind the Magic Kingdom. Are we talking about immediately behind the park, or along the perimeter road that surrounds the park? There's a pretty big difference between these two areas. There is actually a whole lot of stuff around that general area at the north side of the park; some within the park boundaries, and a whole lot outside those boundaries. I'm thinking of the park boundaries as the railroad tracks here.
For example, right behind ToonTown (and I mean behind Mickey's house in this context) is the show building for 2K. You can see it this picture.
http://www.themeparks.com/library-06/wdw/aerial/aerial06.htm
The large building to the right of the tents is the underwater show building for the old 20K ride. If it's in that area, it might likely be connected with the removal of the lagoon.
However, if it's on the perimeter road it could be something like a fire station or any number of other public utility type facilities. I seem to remember seeing a site being cleared on the south (magic kingdom) side of the perimeter road, just to the west of the MK north security gate about 2 months ago but don't remember for sure what it was for.
Edit: Okay I just found this aeral shot that might make what I'm talking about a whole lot easier to understand.
http://www.oitc.com/Disney/WDW/Remote/MagicKingdomLarge.html
You can see the whole service area behind 2K Lagoon that goes over to Pinnochio Village House where the main entrance to the Utillidoor system is. If you follow the train path around, you'll see the vehicle bridge that crosses over the tracks and heads out to the perimeter road and eventually the whole north service area.
So the question is whether the area in question is in this service area, or outside along the perimeter road.