The mine gates were unlocked. This disturbed everyone in the party. One did not leave the door open during an undead apocalypse, after all. At least, not twice.
Carlotta had chosen herself, Jimmy, and Witherby for the expedition to shut down the beacon; Loud Mary and the others she kept outside, on the principle that six would probably not be more likely to survive than three. If the three outside had to flee, say, an awakened horde of shamblers, they could always split up, and increase the chance of somebody making it back to the castle. And she absolutely had to go, since Carlotta was the only arcanist in the group.
Also, she was a duchess, which was a duty that now had expectations attached to it. No doubt her companions might have preferred to have a duke along, instead, but she was here and there were things that needed doing.