My T had a death in her family and I got a call the day before the session we were supposed to resume, from someone else, to say she would be out another week. I didn't know who died and my imagination went haywire worrying about it and identifying with it (my mother died when I was a very young child, part of why I was in therapy).

Now, from 15 years later, I can see that it could have been an aunt, cousin, "minor" family member that would have been sad but not that devastating to her. My T's family was in another country and she could have just had travel problems or been tired from her travel, any number of things unrelated to the importance of the person to her and how she may/may not feel?

I might ask T why he cancelled group; I personally would imagine that it's like individual therapy and "running" group isn't the whole story; your T wants to be there and part of the process as that can't be "made up" when he gets back; he'd miss something vital/what happened in the week he wasn't there. The other T might have had a problem too, unknown to you, maybe had to take more individual clients because of T's absence (and thus would be "overworked") or maybe was wishing for a session/time away too and since T wasn't going to be there, having that time free was a help to other group T, etc.