More accurate (or at least brief) would have been to say "I'm not in." It's unlikely you will find me in my office this semester. I am more than happy to arrange an appointment. Just email me.

If my office door is open, feel free to barge in. If my door is closed (completely shut), please think twice before disturbing me.

If it's urgent, email me and I'll get in touch with you quickly. Note, I go very loooong stretches of time without remembering to check my voicemail.