If user affiliation is marked as persistent (which can be done using admin ad-hoc commands), MUC delivers presence to occupants in name of offline user.
MUC generates presence with
extended away info:
<presence from="…" to="…"> <show>xa</show> </presence>
This presence is sent to occupants, when user goes offline and when persistent occupant is added to room (but he is offline). If persistent user if online in room, then MUC sens real presence of occupant.