Magically Reappearing Files

Like all my users, this user has a mapped drive G: on our network. In keeping with good house-cleaning, all users are require to periodically delete unused files from this drive. For some reason when this user deletes files, one or two days later, they reappear. I thought he was maybe doing something wrong, but I watched him and I even did it myself and they came back. What the heck is going on?