If they hadn't been watching everything on the screens, it would have been easy to miss the point when the spell dropped over one of the attack sites, disrupting communications for a moment. It would have been hard to miss the radio announcement that there was an invading army of magic using humans coming toward the city, but they definitely would have missed the activation of the Dungeon Spell.
Watching it activate in real time gave Wolfe and the others a clear view of how it was being done, and the flaws in the activation of the spell.
They still hadn't put up an outer barrier around it to keep the area isolated, and they hadn't made any attempt to block communications. But with his sense of mana manipulation already on alert, Wolfe was able to feel the change in the distance as one city to the south of them had a point in time marked by the spell, right before a portal to another world opened.