One reason why workplace bullies may not be discovered is because they suck up to the authorities while bullying subordinates and peers behind their backs. Just like Eddie Haskell from the old "Leave it to Beaver" show (who ingratiated himself to the parents while tormenting the Beaver), the bully pretends to be a model employee - but only when the boss is around.
The night before I was to do a full-day leadership skills and team-building workshop with a company, I received a frantic call from an employee who told me about her bully-boss. She was obviously distraught and pretty much at the end of her rope, as she discussed the daily torrent of insults and put-downs that she had to endure from her supervisor. At the first morning break, I was approached by her supervisor. "I think my leadership skills are pretty good, but I really want to improve them." He then went on to tell me about how much he cared for his supervisees - "they're like my own kids." Just like Eddie Haskell. Of course, throughout the rest of the workshop he looked engaged, making eye contact and nodding whenever I was looking his way, but I overheard him talking to another supervisor about "having to sit through this bullshit."