The following is an excerpt taken from here:
If you are not successful in encouraging the victim, directly or indirectly, to file a formal complaint, your legal duties are not over. In fact, they may just be starting. Once you have actual notice under Title IX, your duty to investigate the report is absolute. There are no exceptions. Yet, investigation is a very broad term, and may indicate merely a preliminary inquiry, or it may include a much more elaborate inquisition into the facts. With an anonymous report, your ability to investigate is more limited. You would satisfy your legal duties of due diligence by checking the report against other recent anonymous and formal reports, to determine if a trend or pattern may be apparent. If so, you might decide to take some action based on the composition of assaults, rather than on just the one anonymous report you have received. You may even gain information on repeat perpetrations in a single location, and this may allow you to target a high-risk population or location with enhanced enforcement, patrols, lighting, cameras, timely warnings, etc.
In addition to the duty to investigate, you may have a duty to attempt some form of remedial response, even to an anonymous report. For example, if you learn of multiple perpetrations by the same individual (anonymous reports sometimes include the name of an alleged perpetrator or enough detail for you to figure out who the alleged perpetrator is), or multiple perpetrations at the same event, you may decide to alert the alleged victim to this information to see if this makes him/her more willing to file a formal complaint, or 3you may decide to launch an investigation into the campus event that produced multiple reports (such as a party at which multiple drinks were laced with GHB).