Infernal Creatures: putting off physical form?

Unlike all other realm-creatures, the description of Infernal Creatures says

"All infernal creatures can put off their physical forms and revert to
being pure spirits at will."

(emphasis mine)

Assuming that RoP:I will not throw a wrench in the works and water down
that "all" (though I hope it does!), in the generic ArM worldview, are
such spirit-forms invisible or not?



Unless you have second sight, I'd say so.