Errors at login : in which log can I get the message ?

[Erwan David <erwan@rail.eu.org>]

At login I get the following errors :
[   21.563380] systemd-logind[2908]: Failed to start unit
user@1000.service: Unknown unit: user@1000.service
[   21.563408] systemd-logind[2908]: Failed to start user service:
Unknown unit: user@1000.service

and at logout the following ones

[  329.910312] systemd-logind[2908]: Failed to abandon scope session-2.scope
[  329.910345] systemd-logind[2908]: Failed to abandon session scope: No
such interface 'org.freedesktop.systemd1.Scope' on object at path
/org/freedesktop/systemd1/unid/session_2d2_2escope

I submitted a bug, however I could not find those messages in any log :
why ?

If something is logged to console, it should also be logged to a file,
for an admin to be able to access the messages afterward.