A trick I use when I'm unsure of any obstructions behind a wall is to drill a very small pilot hole that just makes it thru the wall surface. Then I take some sturdy metal wire, insulated electrical conductor or a coat hanger and bend one end into an arc. Now feed that bent end into the hole and twist the coat hanger around to see if it hits anything. You will easily be able to tell if there are any obstructions. I love this method because it will help inform you of any wiring, plumbing, studs or other obstructions. I made a quick image to help explain it better.
