I'm trying to mount a flatscreen TV above our fireplace that was converted by a previous owner (from wood?) to direct vent. During the process of finding the studs, I noticed that magnets stick the wall in a continuous rectangle, not in discreet spots like one might expect drywall nails to be. In the picture below, I put magnets in some places; again, they actually stick in between all of the places.
Notice that the rectangle is off-center with respect to the fireplace.
Here is the picture of the other side of the wall:
Any ideas what might be behind there? And any ideas how I might go about mounting the TV bracket?