If it's a contrail, then FAA should have had the plane on radar.
Well, the FAA doesn't monitor anything of itself; I think you mean that it should have shown up on airport radar for that sector at a particular altitude. (Actually, I'm not sure that it didn't). Large commercial aircraft are tracked using transponders. They're not identified from radar blips. Now, in terms of military surveillance of US airspace, it's likely that it would be tracked --if it entered US airspace from outside.
Fwiw, a contrail doesn't have to be horizontal. It could be formed by an object moving vertically, which would have made it invisible to civilian radar. I.e., a hotshot pilot could push through the sound barrier from takeoff and produce a contrail. However, the military is supposed to announce testing and even routine flights --and give an advisory or issue a 'no fly' warning. But, even a civilian amateur rocketeer would be required to announce the launch of anything into commercial airspace.
"A man is rich when he has time and freewill. How he chooses to invest both will determine the return on his investment."