FWIW, that USER32 entry is identical to one in my system log after automatic updates were installed. But immediately before that there are several events from Windows Update Agent and NtServicePack that I assume you would've noticed.

Is it possible that no events have happened since then that are ones your system is configured to write to the system log? Since you know it was at least logging restarts prior to this, have you tried rebooting then looking for that in the log?