Sagarchalise brought me on the right track, but only half the way.
The place, where to look in Xfce is indeed
I found a lot of files there, including some empty Thunar- files, xfwm4- files and, interesting for us,
asux is the name of my local machine, the hostname.
:0 reminds of the way, the XServer enumerates different instances, which I used rarely, but I used it, so I have a
xfce4-session-asux:1 file as well, and it is rather old, but different sessions aren't stored in different files, but in different sections inside the file.
Such a section looks like this:
So you can see the name ("2010" in this case) of the session, a bunch of key-value-pairs, and as last these pairs a LastAccess key. It stores the seconds since 1.1.1970 UTC. It's not short and easy to translate it to a human readable date with
bash arithmetic (or tell me how in the comments). But at least the age in years is worth calculating:
So it is 41 years after 1.1.1970 (ignoring leap years and daylight saving time, and cutting the result to whole years) which is 2011. The other fields can be computed as well - I prefered to do it with
scala> val d = new java.util.Date (1315125649*1000L)
d: java.util.Date = Sun Sep 04 10:40:49 CEST 2011
So I remove the whole section, and after relogin, this session is vanished from my list of sessions.