If I was out and someone noticed, I would just say "I was out. I'm back now!" And if they persist, the get the exact same answer. When you give the exact same answer mutiple times, they finally get the message that there is a flaw in their question.
Supervisors/bosses do need to know how long you will be out and when you will return, so they can manage the business needs. That is all they need to know.