Well, it was certainly pretty strongly implied that she was the department chief of the communications section, since any time they had a briefing with "all the department chiefs" she was there. If she was on board, regardless of her assigned duty shift, she'd be on the bridge once that red alert siren started going off. So, it's a pretty safe bet she was on leave and away from the ship at the time. Maybe even in the Rigel system (wouldn't that have been a nice bit of added drama to the story?).