storage box on outside rear wall

I'd think an electronic stud finder would find them in your rear wall. Sometimes if the conditions are right you can see them by differences in condensation on the wall, too. You may be able to suss it out by tapping on the wall. If there is a window (or more than one) on the rear wall, there almost certainly would be a framing member on either side of the window(s).

I would not necessarily trust the corner of the roof. It's possible there could be a nice sturdy framing member that they're going into, but it could also be that they simply hold one relatively weak piece of sheet material to another.