Reportedly, he was USSS Agent Todd Madison, her Detail Leader.
Here he was in action, when animal rights activists got out of hand at a Clinton rally in Las Vegas on 4 August 2016:
https://www.youtube.com/watch?v=Heb_whqrGok
He takes charge, and, having ascertained there is no problem, he shoos away the other agents and then fades away himself.
http://www.wnd.com/2016/09/hillarys-mystery-man-spotted-during-blood-clot-hospitalization/