Does Kevlar (in a Flak Jacket) show-up on X-Ray machines???
Not positive, but from watching TV, Kevlar seems to have some mass to be effective. TSA requires coats be removed while screening, and they go through Xray. I doubt it would be invisible.
I guess the question could also be, if suspects are caught at screening with defensive protective items, then arms onboard are increasing the hurdles for a successful attack?