It may not be concrete walls, but they could be cinder block walls for the outer walls, which are hollow. I am not sure if they fill them or not during construction. You could check by using a skinny nail to see if you can nail it all the way in in a discrete spot. Tool Time Tip of the Day: You can fill the hole in with toothpaste instead of traditional compound, which you probably won't have on hand.
Another way to check, similar to the hammer test, is if you have an electric razor, run it along the wall. The sound it makes will change when you go over a stud compared to the space between the studs. If the sound doesn't change, chances are it is solid behind the drywall.