Gah, I hate that, I'm a gun owner, I have a permit, I carry a Glock 36 every day, and every time I get an earful of the classic "gun owners are evil" screed I hear the tale of the all plastic Glock, that no metal detector can see. I tell people there is NO SUCH THING, I show them Glock's website, I get blank stares, they just KNOW its real and I'm just a "bad ol liar" defending my horrible evil ways.
In the end I asked them where they heard about the magic invisible Glock, they just "read it somewhere" or "someone told them about it once" Its way more fun to believe than the actual truth.