I had the same problem here (we are in moodle 2.6.8). This problem is due to this line in the /lib/classes/session/manager.php file :
ini_set('session.gc_maxlifetime', 60*60*24*4);
Indeed, this line forces the max session lifetime to 4 days. You have to replace it with this line :
ini_set('session.gc_maxlifetime', $CFG->sessiontimeout);
The $CFG->sessiontimeout is set in the admin panel (/admin/settings.php?section=sessionhandling).
With this patch, logs are gone
Hope it helped !
Regards,