Well the mystery has been solved. I have found a birth record for Sarah Mitchell who was the wife of Benjamin Foss. I found the record in the Vital Records of Brunswick, Maine. I ended up looking in these records due to a lead in the book Old Kittery and Her Families.
When Israel passed away Mary (Berry) Mitchell remarried Joseph Getchell in 1750. In 1765 both the Foss and Getchell families moved to Machias. When the history of the town was written it was probably assumed that Sarah was Joseph’s daughter. Furthermore, Benjamin Foss and Sarah Mitchell were married in 1762. If she was the daughter of Joseph Getchell she would have been born after 1750. This would have made her 12 years or younger when Benjamin and Sarah got married. This would have been highly unlikely. The only record I was missing was a birth record to prove whose daughter Sarah was. Below is the information from the Vital Records. Interestingly, neither John or Sarah were recorded in the baptism records in Records of the First Congregational Church in Scarborough, Maine. However, some of Israel and Mary’s other children were recorded, which included Mary who was born on May 17th, 1736.
Israel Mitchel and Mary Mitchel and his wife
–1 their Daughter Mary Mitchel was born May 17th 1736
–2 John Mitchel son of Israel Mitchel and Mary and his wife was born October the 22th 1739
–3 Sarah Mitchel Daughter of Israel Mitchel and Mary and his wife was Born Jully the 9th 1742
Anderson, J. C. (2004). Vital records of Brunswick, Maine, 1740-1860 and the Forsaith book of Brunswick family records, compiled 1876-1880 by Jonathan W. Forsaith, town clerk (p. 24). Rockport, ME: Picton Press.