Almost all instant messengers have some type of 'status' that can be set while one is online, frequently with such default settings as "I'm Available" or "Out to Lunch", etc. Why not simply apply the same to when one logs off? A text string could be set that would appear as a status in the normal manner next to the logged off name - this would leave an explanation for your hasty departure to your friends, and could be set quickly enough to keep from further hindering you from whatever the urgent occasion is. Similarly to the online statuses, generally vague default messages ("Something Came Up") to more specific user-defined strings ("Lightning Storm Approaching") would be functional.