I'm not an Information Security professional, just a worried layperson.
A few years ago, when I was logged into my own account on a shared laptop, the disc space mysteriously kept filling up. I deleted vast amounts of videos and other large items to free up memory, but the 'almost out of disc space' messages kept coming. The other user was not logged in at the time. I asked him if he was syncing large amounts of emails or photos to the laptop, which he denied.
The other user is now being investigated for possession of indecent images/videos. Is it possible that he caused the disc space to fill up with illegal content, without being logged in at the time? E.g. by syncing from his mobile device / torrenting to his laptop account?