My father owns a NAS by Synology. I have a sneaking suspicion that it might have been infected by some form of malware. I noticed for example it often increasing in overall and specifically fan noise at odd times and when nobody is using it.
What are tools I can use to scann it for unwanted intruders and if found get rid of them?
I went down that rabbit hole too when I thought it should have been idle. Turned out it was routine data integrity checks and indexing.
Ages ago Synology used to have (maybe still does) a virus scan utility based on clamav (I thiiiiiink) and it only found windows viruses I had in spam emails on my Mac backups.