files disappearing from user directory

I have a an issue with file disappearing. Small office, 5 users, SBS server. Most of one user’s documents disappear every night! I have to restore from backup tape. This only happens to one user, and all the documents are there when they leave in the evening, then gone in the morning. This only happens to one user and she does not log off at the end of the day. The backup occurs at 9pm and all the documents are there then, too. Something happens between 10pm and 7am??? How do you turn on auditing and would it help to determine what’s going on here?