They can rip off their leg and beat people with it. They should have to check that appendage.

I don't think a typical metal detector can distinguish between metal in my arm and metal taped to my arm.

That's why they use the full body scanner at most airport security now. One of those detected a small piece of scrap paper in my back pocket that I had to pull out and show to security. They showed me the image and even the shape of the paper was clear. For sure those scanners can distinguish between metal in your arm and taped to your arm.

Okay, haven't been flying that much in tje last few years, I thought they were still not universal - at least in europe.

Not universal even in the US, but you encounter them fairly frequently in Europe depending on the country.