The body scanners can't distinguish them from something more dangerous. I wore a sequin front tank under a sweater and the girl behind me had a sequin front tank with a blazer and we both got pat downs and our hands swiped and tested for explosives due to how the scanners see sequins. They turn your chest bright yellow on the screens that show the body scan results and therefore prompt further investigation.
Oh my goodness - this happened to me a few months ago. H and I planned to go straight to dinner once he picked me up at the airport, so I wore a sequin cowl neck tank under a cardigan. The agent was friendly - she told me she had done the same thing and swears to never wear sequins again to the airport.