Two people walk into a room. After 30 minutes one leaves. How many people in the room and why?

We don't know how many people were in the room before the two arrived, we don't know how many others arrived or left in the proceeding 30 minutes. We don't know if there's another method of a person not being in a room other than leaving (i.e. spontaneous human combustion, being dissolved in acid, ingestion, metamorphosis). In short, we can't comment with any authority on how many people are in the room.