If you're talking about "plastic guns" like a Glock, the barrel is still metal, and parts of the trigger assembly may be metal as well. If a metal detector at the airport can trip due to a belt buckle, why couldn't it trip due to a gun barrel?
Even more robust DIY guns like this use metal components for only the breech, and that's a very small component that could easily be confused for any number of other things.
Breach only makes a lot of sense. That would indeed be hard to detect, and would likely solve the above.
Apparently the 380 cartridge doesn’t explode upon firing