User blog comment:Katachino/deku’s marking/@comment-25044056-20181215160151/@comment-36815142-20181215220511

Or perhaps it resonates within the current user whenever he is near a descendant of an OFA user that isn't evil because OFA wasn't made for evil purposes, so maybe that's why it resonates near Shinsu and not Shigaraki.

Just a personal thought. Also, the thing regarding AFO hearing his little brother might be the reason Deku has the vestige- to prove that AFO was listening as well? Or because the next morning he would talk to Shinsou? Can't wait for Ch. 211 tho, might explain alot.