Also, get it in writing from the client;  that is, that they know and understand that this work is only from a specific date and covers no other prior matters of public record.  Quote their email as part and parcel to your report and billing, so that you've been repeatedly VERY clear about the limits of such a search.  Also disclose that this may be a practice that they, as a client, orders specificly, but that your firm does reccomment nor consider to be part and parcel to good abstracting practices and that the results of any failure to locate prior owners whose interests were not deeded out or otherwise settled, is not your liability.  A letter of indemnity against such items, could also be signed by the client as an added affirmation of such disclosures.  The more paperwork, the better.  document it, document it, document it, and then repeat again!