If you observe improper behavior that isn't egregious, ask the employees to stop and then report it. Likewise, if a manager opens an office door and finds two people in a lustful embrace, that's a serious thing as well. A mailroom worker is accused of slapping a woman on the bottom. It's a first-time complaint and there are no witnesses. The human resources department or your company's sexual harassment committee needs to conduct an investigation.
HR needs to determine the behavior leading up to the incident and what occurred directly afterward.
The company should speak to other employees in the area to corroborate the statement. In all of these cases, investigators need to know the severity and the pervasiveness of the action. The company must consider the person's record and how other employees in similar circumstances were treated. The reason is that human resources is the central department where all complaints are sent. When you hear an accusation, you may not realize it is the tip of the iceberg, that there have been other allegations against the harasser.