Deleting a file does not overwrite the data. It just marks that area of the drive as “free”. Using forensic tools (or simply dd) you can read data from “empty” parts of the drive. To be save you have to overwrite each file. You can try tools like shred to scramble the data of a file before deleting it, but as they say themselfes, unfortunately your filesystem might not actually let you do that (scroll to the CAUTION section).
You can use dd to write bytes to arbitrary places of the drive, but again, the filesystem might lie to you where a file is actually located. The ONLY way to be entirely sure is to wipe the whole drive. And if your IT does not do that before handing a system to a new employee, then they are not doing their job correctly.
Not legal advice: just tell them you installed a few viruses (while the system was not connected to the company network of course) and they should nuke the system before handing it to anybody else.
(You can also wipe the drive yourself by booting from a stick and overwriting the entire drive)
Dont think its quantum. Its all deterministic and based on the music of the ainur. So its analog.