External hard disk use
Well it appears that the files are cache files and are deleted regularly as part of keeping the drive clean and not cluttered with random thumbnails/logs/whatever other cache files are stored. Its pretty obvious from the entry that operation run is CACHE_DELETE so I wouldn't worry too much.

Also, from the image it says "category: daemon". If you are not aware, a daemon is a background process that takes care of small nitty-gritty things for you, like starting services, clearing unneeded caches, and many many more. You can utilise certain daemons to regularly run tasks for you like cron for example. Its nothing to be worried about and its vital to any operating system.

If you are suspicious of malware, you could regularly check the files on your drive by running du -sh every so often to see the space used on the drive. If there is space free that you did not delete, then it may be the case. There are many other ways to check for malware deleting files theoretically (antivirus, disk space, running processes etc) - but if you do have malware on there without your knowledge, then you have much larger problems than it deleting your cache for you :)