The Devil Banishers announce that they will judge Nero and Marie in public. Julius then orders the Black Bulls and other Magic Knights to go rescue them. The Magic Knights place themselves among a crowd that clearly hates the devil, and then the Devil Banishers appear. They then demand that the devil-possessed Asta show himself.
So it does appear that they can prevent 
their mana from being detected.
.يبدو أنّهم يستطيعون حقًّا إخفاء المانا خاصتهم
Anscheinend können sie verhindern,
dass man ihre magische Kraft erfassen kann.
Poseen algo que oculta su poder mágico.
Poseen algo que oculta su poder mágico.
On dirait bien qu’ils ont un moyen
de déjouer la détection du mana.
A quanto pare sono in grado 
di rendere il loro mana impercettibile.
Então parece que eles são capazes
de disfarçar a própria mana.
Похоже, они всё-таки умеют
скрывать свою ману.