A lot of buyers and sellers think that every home available for sale is in the MLS. That is, sometimes fortunately, not true. Particularly at the high-end of the market, a few homes are made available "off market" or not on the MLS.
These homes may or may not be luxury estates. Some of the reasons for selling a home off market include:
Need for privacy
People who are home a lot and always need appointments
The seller doesn't want "look-i-loos" or tire-kickers
Off-market homes are usually available through a Realtor, especially one that knows the specific area in which you're looking. When looking for a property not on the MLS, ask your Realtor and they should be able to connect with other agents in their office or in their town to find properties that fit your needs.