Use your agent as your resource. There is a list published in MLS of broker's open houses every week. Some are hosted, which means that you would be able to access them, and others are "no host", which means that they can only be accessed by agents who have lockbox keypads. Most new listings have hosted brokers open houses the first week that they are listed, so they are typically available to be seen. Southern Marin and Novato broker's open houses are on Wednesdays, and Central & West Marin broker's open houses are on Thursdays.
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