As a general rule, we ask people not to speculate about where members are. I know we all get curious, but we try to preserve member privacy as much as we can.
We can't really take a position that "we'll lock the threads where a member is gone due to discipline, but keep open the ones where nobody really knows", either. Obviously, that would just let everyone know who has left on their own, and who has been asked to leave.
My best advice is that if you miss a member, reach out to him/her by PM. Obviously, a lot of times people won't be responsive, but it's the best we can do. (Oh, and nothing in our rules stops you from making them feel special when they do come back, if they so choose.)
Thanks for understanding. Locked.