I just moved into a condo that was built in 2006. I recently hired an electrician to come install a recessed outlet in the middle of a wall to prep for a TV wall mount. When the electrician cut into the drywall to put the gangbox in, he found a sheet of 1/2 - 3/4 inch thick plywood behind the layer of plywood that he had to cut through. Does anyone know why that is? Behind the wall there is a staircase going downstairs to a side exit if that makes a difference.

We had difficulty locating the studs in the wall before we decided on the location of the outlet. How would I go about locating the studs without ripping down the entire wall? If this plywood sheet is along the length of the whole wall, is it strong enough to hold up a mount and 60 lbs? Thanks!