A.Sean Kenny responds: First, try to narrow down the approximate location of the hidden box. The National Electric Code ( NEC) requires that outlets be placed no more than 12 feet apart, and that there be an outlet no more than 6 feet from a door opening. Most outlets are located 18 to 24 inches off the floor, and wall switches are generally located about 4 feet above the floor.
Carefully scan the wall surface in these areas for a hump created by the hidden box pushing against the drywall. If you can’t see a hump, place a long straightedge on the wall, and move it around until it rocks over the high spot.
to read the full article.