There are some code paths where a session might be removed without any mention of it in the log. The prominent ones are the periodic poll and get_sessions()/get_usersessions().
It would probably be useful for an admin to see when a session died (or at least when vsmserver noticed).
Basically the same thing when reconnecting or shadowing. You just get the log line:
> User foo tried to reconnect to vanished session
Actually this is a lie. We fixed this in r31963 for bug 5489. You get this in the log:
> 2020-03-10 10:01:05 INFO vsmserver.session: Session 127.0.0.1:11 for tltest has terminated. Removing.
Verified with nightly build.