Just a thought but I found a disk on a work computer once (back in the early 2000s/possible late 90s) with a mysterious chunk of memory missing. Turned out it had been compromised by a hacker and they were using it to store all sort of what I presume was porn. The only way I could see what was taking up the space was by defragmenting the disk and some of the most fragmented files (which were otherwise invisible) were called things like '....Kylie gets hot..' and such
I reformatted the disk and removed it from the network as it just ran an instrument we used in our labs.
I would check firewall and antivirus scan it as a precaution.
(That was Birmingham University's genius security policy back then - every user for themselves - you were responsible for your own computer's online security - consequently I'm confident about half the campus was virus/trojan infested).