The document filed in Essex Registry of Deeds refers to Richard Martin, of course, not Robert. (My error, too late at night.)
Is there any chance the town of Amesbury could have taken ownership of the property? In that case "Amesbury, town" would be listed as the grantor to a new owner, and that might have been missed in previous searches. It would be fun to look at grantor deeds between 1692 and 1700 from the town (maybe Amesbury or Salisbury) to a new owner of property described as "formerly that where dwelled George Martyn (or Susanna, etc.)." I don't know if towns took over property at that time and place, but I bet they did.
Kathleen A. Martin