Security things don't detect them.
I can see X-ray or terahertz scanners missing a tiny lump of metal, but aren't there a fair number of magnetic scanners in use looking for larger lumps of metal, which I'd think the magnet would interact fairly strongly with?
http://www.theverge.com/2012/8/8/3177438/cyborg-america-biohackers-grinders-body-hackers