I'm not sure if the E&O insurance would even cover a search that is done outside the official public records. If an error is made it will be the official public records that will prove the case either way.

I believe a case could be made that  relying on an unofficial source that disclaims accuracy of completeness of its records is in itself an error.

At the very least I believe anyone using these unofficial sources has an obligation to inform the client that the source does not support an accurate or complete report.