I noticed a lot of entries of that type in /var/log/vsmagent.log.
From what I can tell, process 123 indeed doesn’t exist anymore, for a good reason: it corresponds to the previous tl-session, which from a user perspective ended as expected.
The log entry seems to be generated by an obfuscated /opt/thinlinc/modules/thinlinc/vsm/handler_verifysessions.py.
What could be reasons for ThinLinc believing that the previous session should be alive?
When needed, ThinLinc makes sure that its database of running sessions actually reflects reality by checking that the tl-session process owning the session is still alive.
This is required for having a cluster of multiple ThinLinc servers, as the machine keeping track of all the alive sessions (the master(s)) may be separate from the machine(s) actually hosting the sessions (the agents).
In this case, the wording (and WARNING log level) is probably a bit harsh, as this is expected when a user has logged out. There is now an entry for making the logline less scary in our issue tracker: