unity-panel-service --lockscreen-mode not started for legacy users

[Klaus Reichl]

Ubuntu 15.10 (maybe versions > 14.04) show(s) the following:

Existing users do not "blank screen" on idle.
Newly create users do.

Analysis showed that:
Old users do not have "unity-panel-service --lockscreen-mode" running
while the others have.

Starting "unity-panel-service --lockscreen-mode" manually fixes the problem temporarily, however, after logout/login it is gone again. (Please don't tell me that it could be implemented in .config/autostart - newly created user do not have this either!)

This means "/usr/share/upstart/sessions/unity-panel-service-lockscreen.conf" is not considered for a legacy user for whatever reason. Question is why? Any hints?