by Ken Task.
Ok, understood now! That's really strange! Autobackups take many settings from backup defaults for the site and we can see .nu. - no users - in those.
But backup-moodle2-activity-261-261-20190409-0414.mbz - doesn't indicate nu in the file name plus it appears to indicate activity 261.
Since that file is outside 'the sea', copy one to a test directory and uncompress it. Look at the moodle_backup.xml file as well as other xml files for some clues - like the course id for the activity.
Better yet, user that might have created it ...if present.
Got any additional plugins?
Indeed strange behavior ... at least to me. Others welcome to chime in here!
'spirit of sharing', Ken