We've actually hit this before. "Out of Character", Nichols was not a regular cast member
, but a 'day player' - a few steps up from an extra, but not by all that much. This limited the amount of time she could have on screen, due to union rules (which were and are very strict about such things).
In character, she simply wasn't on shift for the few hours while this was occuring. With her station already covered, when the red alert sirens blare, she had damn well
better be at quarters, as regulations dictate.