Plastic magazines aren’t undetectable, they still need metal springs.
It's possible that the mass of the metal spring would be insufficient to trip a metal detector. Not sure how sensitive they keep them.
IIRC, those printed plastic ‘springs’ didn’t last very long. And I think the feed lips also had some cracking problems.
And they'd be detectable when they were loaded. Unloaded they pose no danger.
http://www.google.com/#hl=en&tbo=d&sclient=psy-ab&q=plastic+springs+material&oq=plastic+springs&gs_l=hp.1.1.0l3j0i30.0.0.1.500.0.0.0.0.0.0.0.0..0.0.les%3B..0.0...1c.p5N_508x6VM&pbx=1&bav=on.2,or.r_gc.r_pw.r_cp.r_qf.&bvm=bv.41248874,d.b2U&fp=e33da1aae51909f2&biw=1600&bih=778 ~ huge bunch of things about plastic springs.
Most use them, but they don't NEED them. A pneumatic system or external rubber elastics would work.
Nothing is undetectable regardless of metal springs or not.
Airport scanners can see everything in your luggage, including a used condom if you should happen to leave one.......
A few years ago I applied for a job with the TSA and went thru their testing program which included identifying certain objects which were photographed from actual xray images from baggage scanners.
Of course you don't see the images in 3-D but rather the outlines of every article in the piece of baggage. And with practice and training, certain shapes will require a personal inspection of that piece of luggage........
Remember the government outcry years ago when the plastic Glocks first came on scene? Liberals all had their panties in a wad claiming that people would be able to carry these firearms on planes undetected...........