There are 2 ways off-market property pages are generated on Trulia.
First, third party data providers compile public record information from the local assessors, tax collectors, and recorders offices across the United States going back for a period of 10 years. When this data is sent to Trulia, it creates an off-market property page containing the public record information. This data is regularly updated automatically as we receive it from these data providers.
Secondly, an off-market property page can be generated after a previously active listing posted by an agent or homeowner has been taken off of the market. This type of property page does not include public record information since it was user created. These pages typically host less information about the home.