>I have a garage where the drywall isn't attached directly to the studs at all.

I almost went insane not finding the studs in my garage! As it turns out they layed what seems to be 1"×4"×8' boards horizontally over the studs, then drywall over those. So if you run a stud finder horizontally you'll never find them because there's an air gap between the stud and the drywall.