It's been known for some time that there are pocket listings and "off market" listings. In the last couple of years, following the real estate collapse, this "grey" market has really taken off. Especially when it comes to commercial real estate, is now including residential properties.
A short fact: Realtors are required to list their listings on the MLS within 3 days. But there is a clause in the listing contract that allows an agent to keep a new listing off the MLS if the Seller chooses so. The key word though is "Seller". An agent can not do it him or herself without the Sellers knowledge.
As Realtors (agents and brokers) get listings, many of those listing do not get sold and expire. This is where the "underworld" can kick in. There are those Realtors who will contact the owners of properties that had their listing expire and try to get them to relist with them. Savvy agents will either contact a Seller right away and offer to market the owners home for a price or commission amount if they bring the owner a buyer or wait a few days to a few months and then contact an owner and pitch them the offer of bring a buyer but not asking for a listing. The reasoning behind this is that many owners/sellers get frustrated with their prior agents/brokers lack of bringing a buyer or one of several other reasons. This opens up the "hidden market" or "off market" real estate.
This then opens the question of, "are there bargains to be found in this Off Market or Hidden Market". The answer is mixed with yes and no. Sellers are not tied any one brokerage and they negotiate the terms and selling price. The other thing is that buyers have time on their side to pick up deals as sellers wait longer and longer when trying to sell their properties themselves.