doesnt matter if i am teaching or not - my prep time is MY prep time. as long as i do not partake in any foul behavior or misconduct, i can use an hour in which i have no students (an hour in the day allotted to teachers to reduce the amount of work we take home.)
i live on my own and enjoy doing my work at home - so i am very prepared every day which allows me a nice break in the middle of the day.
see - as adults we learn how to manage our time. if i were you i would concern yourself exclusively with how you spend yours.