How to locate the metal framework?

Although this isn't what you asked, I'll tell you what I did when I installed my shelving. Instead of working around the studs I just figured out the best placement for the brackets to properly hold my shelving and then drilled a small pilot hole first, for each screw location. If I ran into a stud then I just used a smaller sheet metal screw and put it directly into the stud, otherwise I used the small hollow-wall mounting screw. It worked great and the stud-mounted sheet metal screws are more secure and better load handling than the hollow-wall, in my opinion.

Walk outside early one morning and look at the cool dew collecting on your Pod, you can see the outline of all the framework then and do a sketch for your Pod.