Secret World of Secret Investors Buying Secret Houses! (Really?)

[Hjiorst Fjioords]

Please excuse the hyperbole. It is the extreme, paranoid version of something I have been wondering about: Are there properties out there up for sale that are not listed and which won't show up on Zillow? 

This idea occurred to me while reading discussions about hard money lenders, etc.  I wondered if there are properties that some owners or brokers might not want to list publicly, and only offer to their group of associates and those in their private "Buyers" lists. 

I do not think this is likely.  It seems to me that any seller would want as many people as possible bidding on a property.  Then again, I once attended a trial which involved an agent not listing a property (not properly, anyway), and offering it only to a favorite investor client.  The trial did not end well for the agent.

So are there such private groups?  Is there some advantage to not listing?  Could any agent do this while maintaining their responsibilities to their client?