Open Communication about Wrongdoing
Noisy environments are healthier than silent ones. Republics are louder than dictatorships.
Most investigations of wrongdoing initiate from tips - someone near the problem has smelled something funny and reported it. The success of Crime Stoppers arose due to its anonymity and reward system, and in the aftermath of the massive systemic frauds in the early 2000's and Sarbanes-Oxley organizations were required to implement anonymous reporting hotlines.
Fifteen years later, we more often than not still see improper use of company hotlines. Either the organization receives too few allegations, or investigators are swamped with 900 allegations per quarter of "someone is parking in my space."
Let's address the first problem first - people are not using the hotline. Why not? Do people not trust it? Do they not know about it? Or maybe this is not even a big deal, because the company has opened up other communication lines which are proving effective.
The simplest problem to handle is awareness. Promote the hotline with signs in the breakrooms, discuss it in your Code of Conduct and New Employee Orientation, and occasionally discuss it throughout organizational meetings.
In my previous company, our marketing department created a video with the CEO and me discussing the hotline, with examples of its proper use. The video was displayed prominently on the company intranet, and an email was distributed to the entire organization linking the video as well as the Code of Conduct to remind employees to report possible wrongdoing through the proper channels.
Then there is the opposite problem - too much noise. We cannot have people loudly accusing anyone they don't care for, and we also do not want people flooding the hotline with garbage. Accept that we will always receive bogus allegations, because we want to err on the side of communication versus silence. However, organizations can effectively limit the problem by stating up front that leaders expect people to be responsible with the hotline; it is not meant for therapy when they do not like their boss or their situation in life. It is helpful to demonstrate some examples of what you expect to hear on the hotline.
The hotline is not the only reporting mechanism; in fact, I like to see multiple and possibly redundant communication lines open for employees to report allegations. Not too many; we do not want to confuse people or increase the risk that the allegation does not get to the proper department. Some options: Head of Internal Audit, Special Investigative Unit, Legal, Human Resources (not necessarily for fraud, but for harassment, etc.) and the Chairman of the Audit Committee.
Allegations will arise from insider tips, third-party tips, routine audits or inspections, managers following up on odd transactions, customer complaints, and data analytics. Make sure there are mechanisms for capturing all sources.