Disk space consumption moodledata/trashdir after CLI backup

Hello. Version 4.1.1

When I run a backup of the course to a specific directory from CLI 
/usr/bin/php /usr/share/nginx/html/moodle/admin/cli/backup.php --courseid=6 --destination=/root/course_test_backup

For some reason the moodledata/trashdir directoryalsoincreases by thesize of the backupfile.

Please tell me:
1) How to understand what exactly this file is? (The contenthash could not be found in the database in the mdl_files table)
2) How to clean this directory manually (without cron) correctly and safely, do I need to do something in the database?