Did you even give her* a second thought when you saw her sitting in the staff room? Are you aware that she is probably the most imaginative person in the building? In the past week she has taught year 4s about Olmec gods, she has taught year 9s the future tense in French and delivered a science lesson about genetics to year 10s as well as teaching a few lessons on her own subject (maths). As well as being a qualified teacher she is also an executive coach who runs mindfulness trainings.
What about her life outside the school? While you were vegging out in front of the TV at the weekend she could have been helping to lead a retreat or on a workshop working on her own self mastery. Maybe she supports meditation teachers in the evenings and is in the process of editing her first novel.
Perhaps you were off sick and she was covering your lessons. Did you consider the work you left for her or did you just set a page from the text book or print off some worksheets? She probably cannot log onto the computer to find more resources to help her so she has to come up with something off the cuff.
Next time you see an unfamiliar face in the staffroom go and introduce yourself. Ask what they have to offer your school. You may be surprised.
*Of course this equally applies to men. I happen to be female.